Observing Programmes: Stars and their environments

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Guaranteed Time (Key Programmes)


Mass Loss from Evolved Stars (MESS)

Old stars eject their outer layers as the age, forming shells of material containing dust and molecules.  Despite decades of study, the formation and evolution of these shells is not completely understood.  MESS will use PACS and SPIRE to observe over 100 old stars, and will look at over 50 with the spectrographs to identify the molecules and dust components present in them.  In addition, MESS will observe the remnants of five recent supernovae (from within the last 1000 years), left over when massive stars have died in violent explosions.

Click here to go to the MESS website.

Lead Scientist: Martin Groenewegen (University of Leuven)
UK contact: Mike Barlow (UCL)

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HIFISTARS: The physical and chemical properties of circumstellar environments around evolved stars

HIFISTARS will use the HIFI instrument to obtain very high-resolution spectra of the environments around old stars.  This will allow the detection of common molecules, such as water and carbon monoxide, and provide information on the physical conditions and chemical composition of the material surrounding the stars.  The high resolution of HIFI allows the velocity of the gas to be measured, providing information on the dynamics, structure, and evolution of the objects.

Click here to go to the HIFISTARS website.

Lead Scientist: Valentin Bujarrabal (Observatorio Astronomico Nacional)

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Guaranteed Time (Round 1)


Structure and Evolution of Disks around Brown Dwarfs and Very Low Mass Stars

We propose a program of deep PACS photometry of 50 young brown dwarfs and YSOs at the hydrogen-burning limit, 0.08 solar masses. We have selected these from documented BDs in the literature that have been observed to have circumstellar disks based on their 1-24um SEDs. None of these objects is bright enough to have been detectable with Spitzer beyond 24um. We eliminated objects from our list that are likely to be detected in the large-area Herschel Key Project surveys of star-forming regions, and we also eliminated objects whose disks are likely to be below our detection limit of a few mJy at 70um. We expect to be able to detect or obtain important upper limits on the cold disk emission at 70um for all 50 sources, and to obtain useful detections or limits at 160um for at least a third of our objects. We will combine our results with those from Spitzer and ground-based telescopes and then model the SEDs with two different radiative transfer codes existing at the University of Kiel and University of Grenoble. These results will provide the first quantitative assessment of the structure and mass of cold dust around the lowest mass objects that form like stars. Our data and modeling will provide important new constraints to the understanding of disk evolution and planet formation by extending the range of central object masses to ~ 0.01 solar masses.

Lead scientist: Paul Harvey

Allocated Time: 46.0 hours

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Mapping the distribution of the crystalline silicate forsterite in the Cat's Eye Nebula (NGC 6543)

We propose to observe a 5x5 oversampled map wavelength centered on the 69micron band of crystalline silicate forsterite for the Planetary Nebula NGC6543. This will be the first time that we can resolve the location of crystalline dust in a mass-loss outflow. This will help is investigate the dust-formation in the circumstellar environment of an evolved star and look at the link with mass-loss and possibly binarity. The total observing time is 3.87h.

Lead scientist: Ben de Vries

Allocated Time: 3.9 hours

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The rich mineralogy of the post-Asymptotic Giant Branch star MWC922

We propose to observe a full SED of the post Asymptotic Giant Branch star (post-AGB) MWC922. ISO spectra have already shown that this object has a very rich mineralogy. With a full spectral scan we want to get a complete inventory of the solid state bands in this object. MWC922 is one of the few sources that shows a narrow band at 65 micron that can be assigned to enstatite, and as such could serve as an important template to establish the signature of cold enstatite in circumstellar environments. MWC922 also shows a broad band centered around 65 micron, whose nature is under discussion but could be diopside. This could offer the rare occasion where cold diopside could be studied. Solid state bands in the spectrum will be identified using laboratory data available in the literature and we will establish relative abundances of the materials. Furthermore, the presence of the narrow 65 micron enstatite band can be verified and we can search for other enstatite bands in the 65-75 micron wavelength range (clino and ortho enstatite).

Lead scientist: Ben de Vries

Allocated Time: 1.0 hours

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SUCCESS: A SUbmm Catalogue of Circumstellar Envelopes of StarS with Herschel/HIFI

We propose to perform a biased survey of Circumstellar Envelopes (CSE) in AGB and post-AGB stars in order to build a reference catalogue in two to three sub-millimetre and FIR CO transitions. The proposed observations will provide access to the emission of the envelope gas layers in the temperature range ~100 to 2000K. These warm layers are fundamental to the field of stellar evolution because they are the regions where the acceleration of the gas takes place, and therefore where the envelopes are being formed. Because of the significant atmospheric opacity affecting ground-based observations at the corresponding wavelengths, this warm gas could never be probed in a systematic manner over large source samples. In fact, in some of the transitions emitting in this temperature regime, no observation at all is possible from the ground.

The catalogue we are proposing to build will collect the emission of two CO transitions (J=5-4 and J=9-8) probing different layers of the warm gas in about 74 sources, and measure a high-J CO transition (J=14-13) in the 10 most intense sources of the sample. The total time to build this data-set is 35 hours. When eventually combined to the smaller sample collected by the GTKP HIFISTARS, this will form an unprecedented catalogue of over 100 AGB and post-AGB stars in two to three fundamental CO lines of the sub-millimetre and FIR domain, and offer a legacy data-base for the modelling of the shaping parameters of evolved stars.

Lead scientist: David Teyssier

Allocated Time: 35.0 hours

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Forsterite dust in the circumstellar environment of evolved stars

PACS range spectroscopy will be performed around the 69 micron forsterite dust feature of evolved stars. With the spectral resolution and sensitivity of PACS we will be able to fit the profile of the forsterite feature which is very sensitive to temperature. By studying the forsterite feature in a wide range of evolved stars and from different populations (including Galactic Bulge and LMC) we want to determine the role of forsterite in the dust formation in the circumstellar environments.

Lead scientist: Joris Blommaert

Allocated Time: 10.6 hours

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A Molecular Line Survey of IRC+10216

We would like to perform a systematic line survey of the prototype carbon-rich AGB star IRC+10216. This object is particularly rich in carbon chain molecules and radicals and shows an impressive HCN spectrum in the submillimeter and far-infrared domains. The coverage of the HIFI instrument will permit to characterize the complete molecular content of the innermost zones of the envelope and complement the low frequency observations available from ground-based radio observatories. The sensitivity of the survey is such that at 1 THz we approach the detection limit of a ground based telescope such as the 30m IRAM radiotelescope after several hours of observing time. The 1500 GHz we want to cover requires 45 hours of GT observing time. A line survey covering 100 GHz with the 30m IRAM radiotelescope at an equivalent sensitivity requires several weeks of observing time.

Lead scientist: Jose Cernicharo

Allocated Time: 45.0 hours

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A sensitive search for light hydrides towards IRC+10216

Among the molecular species typically observed in the interstellar and circumstellar medium, light hydrides have been traditionally elusive to detection since their rotational spectrum lies in the submillimeter and infra-red ranges of the electromagnetic spectrum, which is very difficult to observe from ground due to severe atmospheric absorption. This project aims at utilizing the remarkable capabilities of the HIFI instrument on board the Herschel Space Observatory to perform a sensitive search for several light hydrides toward the circumstellar envelope of the carbon-rich evolved star IRC +10216, which is one of the richest molecular sources in the sky, with more than 70 molecules observed to date.

Lead scientist: Marcelino Agundez

Allocated Time: 15.0 hours

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Unveiling the evolutionary paths of the most massive stars

Several important questions remain open regarding the latest stages of evolution of the most massive stars, in particular regarding the exact evolutionary paths taken between the various subtypes of O stars, LBVs and Wolf-Rayet stars, and the mass-loss history of these objects throughout their lives. In this proposal we will address these questions by extending the massive stars programme of the the MESS GTKP. In the MESS programme, we focus on the LBV phenomenon. Here we will focus on the properties of nebular ejecta from a peculiar O-type star and a Wolf-Rayet star. These together will allow us to study the relationship between these three types of related, massive, mass-losing stars, in particular to better understand how the mass loss both changes with evolutionary stage and how it may dictate the evolutionary paths taken.

Lead scientist: Pierre Royer

Allocated Time: 2.1 hours

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V838 Mon: aftermath of a stellar merger

V838 Mon is one of the most enigmatic objects observed in stellar astrophysics in recent decades. It came to attention when it underwent a powerful eruptive outburst in Jan. 2002, increasing in luminosity by a factor of 100 over a period of 3 months. Immediately following this event a spectacular light echo was formed from the outburst light reflecting off the surrounding dust. Hubble Space Telescope images following this expanding light echo have brought V838 Mon to public attention. What makes V838 Mon an unusual star is that the outburst is not of any type heretofore seen; the scenarios of a nova-like event or thermonuclear runaway have been discounted, on the basis of the stellar type and outburst details. The theories that best explain the outburst are a giant star engulfing a planetary system or, more likely, a merger between a very low mass star and a very young, maybe pre-main sequence low-intermediate mass star.

Observations show that the envelope of the star expanded in response to the stellar impact, and than it may now be beginning to contract. Many O-bearing molecules, dust, an SiO maser and possibly a jet have been observed from the star. If the outburst was indeed caused by the merging of two stars, this is an extremely rare event. Herschel observations of this star will allow us to model the kinematics, chemistry, temperature and density structure of the stellar photosphere and the cool envelope surrounding the star. These will help answer questions still remaining about the stellar impact, and will also allow us to understand more about how a star responds to such a violent event.

Lead scientist: Katrina Exter

Allocated Time: 7.3 hours

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Tracing Remnant Gas in Planet Forming Debris Disk Systems

Recent studies of gas emission lines with Spitzer and sub-millimeter telescopes have shown that 10-100 Myr old stars with debris disks have too little gas left to form Jupiter like gas giant planets. Whether enough gas remains in these systems to form ice giant planets is still unanswered. The [OI] emission line at 63 micron is one of the most sensitive tracers of gas mass in the ice-giant region of 10-50 AU in disks, and Herschel PACS is therefore uniquely suited to trace the remnant gas in planet-forming disks.

We propose to obtain PACS line spectroscopy of [OI] (63 micron) for two nearby young stars, HR 8799 and HD 15115, which are two systems with detected giant planets or signs of planet formation, while still harbouring prominent debris disks that could be in the process of forming ice giants such as Neptune and Uranus. The proposed observations will probe down to gas masses of 0.01 Earth masses, and allow us to constrain prospects for ice giant formation, measure gas-to-dust ratios in evolved disks to compare with planet formation / disk evolution models, and put constraints on whether the dust dynamics in these systems is driven by the remnant gas or by the radiation.

Lead scientist: Vincent Geers

Allocated Time: 2.5 hours

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Guaranteed Time (Round 2)


Mapping the distribution of the crystalline silicate forsterite in the Cat's Eye Nebula

Herschel will be able to measure the crystalline dust at different positions in the outflow of the famous Cat's Eye Nebula (NGC 6543). This will help astronomers examine how crystalline material is actually formed around evolved stars and how this formation process depends on the mass-loss properties of the star. This will help astronomers examine dust formation around evolved stars, and investigate any links with the speed at which mass is lost from the star, and the possible existence of a binary companion.

Lead Scientist: Ben de Vries (KU Leuven)

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The rich mineralogy of the post-Asymptotic Giant Branch star MWC922

The ISO satellite showed that the evolved star MWC922 has a very rich mineralogy in its environment, in particular enstatite (MgSiO3) and diopside (CaMgSi2O6). This provides an opportunity to study the formation of such minerals in circumstellar environments.

Lead Scientist: Ben de Vries (KU Leuven)

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SUCCESS: A SUbmm Catalogue of Circumstellar Envelopes of StarS with Herschel/HIFI

This project will build a catalogue of carbon monoxide (CO) emission in the circumstellar envelopes around old "Asymptotic Giant Branch" (AGB) stars, which are stars like the Sun netering old age as red giant stars. This will probe the relatively warm regions of the envelope at temperatures of 100-2000 K. These warm layers are crucial to the further evolution of the star, as they are where the gas is accelerated to form the circumstellar envelope. The catalogue will cover 74 objects, which will give a total of 100 when combined with HIFISTARS.

Lead Scientist: David Teyssier (ESAC)

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Forsterite dust in the circumstellar environment of evolved stars

The mineral forsterite (Mg2SiO4) has an emission line at a wavelength of 69 microns, the intensity of which is very dependent on temperature. By studying this emission line with PACS in a range of evolved stars in a wide range of environments, from the bulge at the centre of our Galaxy to the Large Magellanic Cloud, astronomers will try to study the role of forsterite in dust formation around evolved stars. Forsterite is the most abundant mineral in the upper layers of the Earth's mantle.

Lead Scientist: Joris Blommaert (KU Leuven)

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A Molecular Line Survey of IRC+10216

The carbon-rich star IRC+10216 (also known as CW Leonis) has an envelope which is chemically quite rich, particulary in carbon-chain radicals (CnH). Hydrogen cyanide (HCN), is one of the most abundant species in this circumstellar envelope. Studying this object with HIFI will provide the full census of hydrides and other light species produced through the envelope following different chemical processes. The submillimeter and far-IR observations obtinaed with HIFI will produce a complete inventory of the innermost regions of the circumstellar envelope.

Lead Scientist: Jose Cernicharo (U. Michigan)

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A sensitive search for light hydrides towards IRC+10216

The carbon-rich star IRC+10216 (also known as CW Leonis) has an envelope which is chemically quite rich, with more than 70 molecules discovered so far.  Observations of the light hydride molecules are difficult from the ground due to absorption from the Earth's atmosphere, but using HIFI some these elusive molecules should easily be seen.

Lead Scientist: Marcelino Agundez (Observatoire de Paris-Meudon)

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Unveiling the evolutionary paths of the most massive stars

This project is an extension of the MESS project, but focuses on different types of evolved star; in particular the nebula resulting from ejections from a perculiar O-type star, and a Wolf-Rayet star that is rapidly losing mass.  This will allow a study of three different types of evolved, mass-losing stars, in particular how the evolutionary stage of the star affects the mass loss, and vice-versa.

Lead Scientist: Pierre Royer (KU Leuven)

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V838 Mon: aftermath of a stellar merger

V838 Mon is one of the most enigmatic objects observed in stellar astrophysics in recent decades. It came to attention when it underwent a powerful eruptive outburst in Jan. 2002, increasing in luminosity by a factor of 100 over a period of 3 months. Immediately following this event a spectacular light echo was formed from the outburst light reflecting off the surrounding dust. Hubble Space Telescope images following this expanding light echo have brought V838 Mon to public attention. What makes V838 Mon an unusual star is that the outburst is not of any type heretofore seen; the scenarios of a nova-like event or thermonuclear runaway have been discounted, on the basis of the stellar type and outburst details. The theories that best explain the outburst are a giant star engulfing a planetary system or, more likely, a merger between a very low mass star and a very young, maybe pre-main sequence low-intermediate mass star. Observations show that the envelope of the star expanded in response to the stellar impact, and than it may now be beginning to contract. Many oxygen-bearing molecules, dust, an SiO maser and possibly a jet have been observed from the star. If the outburst was indeed caused by the merging of two stars, this is an extremely rare event. Herschel observations of this star will allow us to model the kinematics, chemistry, temperature and density structure of the stellar photosphere and the cool envelope surrounding the star. These will help answer questions still remaining about the stellar impact, and will also allow us to understand more about how a star responds to such a violent event.

Lead Scientist: Katrina Exter (Instituto de Astrofísica de Canarias)

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Mapping the distribution of the crystalline silicate forsterite in the Saturn Nebula (NGC 7009)

We propose to make a map of the Planetary Nebula NGC 7009, also known as the Saturn nebula. The wavelength of the observation is centered on the 69micron band of crystalline silicate forsterite. This observation will strengthen the results obtained for the PN NGC 6543 (the Cat's Eye nebula). For the Cat's Eye Nebula we have already seen that the forsterite is confined to the inner part of the outflow, therefor linked to the denser last mass-loss phase. The Saturn Nebula is similar to the Cat's Eye in size and age. Combining the observations of NGC 7009 with NGC 6543 will help to further investigate the dust-formation in the circumstellar environment of evolved stars and look at the link with mass-loss and possibly binarity.

Lead Scientist: Ben de Vries

Allocated time: 2.1 hours

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Unraveling the unexpected wind deceleration in the envelopes around evolved stars

At the end of their lives, low and intermediate mass stars lose a significant fraction of their mass via a copious mass loss through cool, low-velocity winds (with speed of around 10 km/s). After several decades of study, the exact mechanisms triggering this mass loss are still not understood in all detail, but it is generally accepted that pulsations and dust formation play a crucial role. Using these two ingredients, the wind acceleration in the envelope surrounding the stars can be predicted. HIFI has the unique capability to study the line profiles of many molecules excited in the circumstellar envelopes, hence enabling us to assess the wind velocity profile in evolved stars. While the HIFI data indeed show for some targets an acceleration of the wind, the widths of the lines in two oxygen-rich evolved stars are larger for the high-excitation lines compared to the low-excitation ones, suggesting a deceleration of the wind. This wind profile is completely unexpected, and can currently not be explained. However, the current dataset at hand is too scarce and not well sampled in terms of excitation energy levels. We will use HIFI to explore the emission from silicon oxide (SiO) in order to to deduce with high accuracy the wind acceleration/deceleration in two oxygen-rich evolved stars. This will enable us to clarify the role of dust formation and pulsations in the onset of the stellar wind around low and intermediate mass stars.

Lead Scientist: Leen Decin

Allocated time: 9.9 hours

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Cold dust in the ejecta of SN 1987A

The discovery of galaxies at redshifts greater than 6 that contain very large quantities of dust leaves little room for dust enrichment by low and intermediate mass stars. A potential alternative source is dust formation in the ejecta of core collapse supernovae, where 0.1 to 1 solar masses of dust could form according to theoretical scenarios. Studies at mid-infrared wavelengths of type II core-collapse supernovae in nearby galaxies have to date not confirmed these predictions. Recently, we have detected a source coincident with SN 1987A with PACS and SPIRE at 100, 160, 250 and 350 µm as part of the HERITAGE key program. Fits to the spectral energy distribution yield a dust mass of 0.5 to 1 solar masses, within the limits set by the predicted abundances for heavy elements in the ejecta.

We propose detailed follow-up observations of a 10x10 arcmin field around SN 1987A. These observations will yield higher angular resolution and higher sensitivity detections of SN 1987A at wavelengths between 70 and 500 µm, which will enable us to make the most accurate SED and ejecta dust mass achievable with Herschel. In particular, these observations will yield the 500 µm flux density (for which we only have an upper limit from the HERITAGE observations) and a crucial 70 µm flux density, which will help to bridge the spectral gap between the mid-infrared peak from the shocked ring around SN 1987A and the far-infrared emission peak from the cold ejecta dust. Finally, comparison between the HERITAGE observations and the proposed follow-up observations at 100, 160 and 250 µm will enable us to detect any evolution in the flux densities from the remnant over a two-year time span. Such repeat observations are necessary to determine whether embedded radioactivity or the ambient interstellar radiation field is the dominant heating source of the dust.

Lead Scientist: Maarten Baes

Allocated time: 2.9 hours

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MESS SPIRE-FTS spectroscopy of evolved sources having MESS PACS spectroscopy

The SPIRE collaborators in the MESS key programme will use SPIRE to enable the acquisition of complementary 194-670um SPIRE FTS spectroscopy of 19 MESS evolved star targets for which PACS spectroscopy has already been obtained or scheduled. The MESS programme gathered SPIRE spectra to be obtained for 23 of the 51 evolved stars for which PACS spectroscopy was performed. We will remedy this situation by obtaining complementary SPIRE spectra for most of the remaining targets that (a) have MESS PACS spectroscopy; (b) are bright enough for sensitive spectroscopy to be acquired at SPIRE wavelengths; (c) are not being observed with the SPIRE spectrometer by any other Herschel programme.

Lead Scientist: Michael Barlow

Allocated time: 15.0 hours

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Dust mass-loss on the RGB: reaching the limits

Stellar mass loss is the main characteristic of the late stages of stellar evolution. Almost all stars with masses between 1 and 8 solar masses pass through the Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB). Mass loss on the AGB has been relatively well studied and is the main subject of the MESS key programme. However, it is known from Horizontal Branch morphology and other indicators that about 0.2 solar mass is lost in the RGB, and for stars of low initial mass this dominates the mass lost on the AGB. Relatively little is known about the mass loss process on the RGB. It has been established that mass loss in the form of dust is present at the tip of the RGB, but little is known about the exact conditions under which dusty winds develop near the tip of the RGB, mainly because the expected excess is small. On the AGB the mass loss is strongly related to pulsation, and also the stars at the tip of the RGB that show the largest mass-loss rates show pulsation.

We will obtain PACS and SPIRE images for a very well defined sample of RGB stars with accurate distances for which ground-based optical and NIR photometry are available. Ground-based high resolution optical spectra that we will obtain and the optical+NIR photometry will very precisely fix the flux from the central star. We will be able to detect the FIR excess if it is present, thanks to the accurate absolute flux calibration of PACS and SPIRE. We will be able to detect total mass-loss rates as low as 1.e-9 solar masses per year. This would push the existing limit on accurate mass-loss rate determination of RGB stars by an order of magnitude, and will allow us to say how prominent dust mass-loss is on the RGB and its dependence on stellar parameters, thereby potentially improving on Reimers law like recipes in standard use in stellar evolutionary models.

Lead Scientist: Martin Groenewegen

Allocated time: 7.5 hours

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Spectroscopy on the bow-shock of CW Leo

Herschel and Galex imaging have revealed the presence of a bowshock around CW Leo located at about 8 arcmin from the central star. The Herschel imaging data have been modelled by our team as part of a previous programme to show that the PACS and SPIRE continuum emission can be modelled by a modified blackbody with a temperature of 25 K. Follow-up IRAM observations on the apex of the bowshock reveal that carbon monoxide (CO) is detected, at a velocity very different from the systemic velocity of CW Leo.

In this follow-up proposal we plan to obtain a deep FTS spectrum and PACS line observation on the OI and CII cooling lines on the apex of the bowshock. If other CO lines will be detected we would be able to derive the rotational temperature near the apex of the bowshock which would reveal the physical conditions there.

Lead Scientist: Martin Groenewegen

Allocated time: 4.0 hours

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Faint Extended Dust Envelopes of Young Planetary Nebulae

We propose to trace the distribution of cold dust in the extended envelopes of a selected sample of young Planetary Nebulae (PNe). Information on the mass-loss and overall envelope ejection process of Asymptotic Giant Branch (AGB) stars is imprinted in the morphology of the extended dust shells formed throughout the AGB phase. In particular the origin of asymmetrical PN shapes and their relation to spherical mass-loss presumed to occur on the AGB phase can be illuminated upon. We propose to use PACS to follow-up on AKARI/FIS observations of young PNe to study their mass-loss history. Only Herschel's unprecedented spatial resolution and sensitivity in the far-IR can detect the faint extended cold dust emission in these objects.

Lead Scientist: Nick Cox

Allocated time: 4.0 hours

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``Eyes and Bullets'': A study of peculiar bow shocks and dust arcs around AGB stars

The final evolutionary stages of stars are largely determined by ejection of mass from the stellar envelope. The mechanisms at work are thought to be understood in broad terms, but several major aspects remain elusive. In particular the transition from circumstellar matter to interstellar matter is poorly known. It has now also become clear that the interaction of mass outflows with the pre-existing interstellar medium on the one hand substantially complicate the interpretation of observational data, but on the other hand provide a new tool to study the interstellar medium. This proposal is dedicated to the investigation of peculiar types of extended structure c.q. bow shock emission and dusty arcs observed around AGB stars. We will obtain deep PACS 100 and 160 micron scan maps of so-called ``bullet'' and ``eye''-shaped interaction zones to study the infra-red emission at high angular resolution and model the bow shock SED to probe its dust and gas content.

Lead Scientist: Nick Cox

Allocated time: 4.3 hours

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Far-Infrared Emission from Bow Shocks around Runaway OB stars

Runaway OB stars are massive early-type stars that travel through interstellar space with an anomalously high velocity. As the star travels with supersonic velocity through the interstellar medium matter is swept up resulting in a bow shock. The exact shape and conditions (density, temperature) of such a bow shock region depend critically on the stellar properties (such as velocity and mass-loss rate) as well as on the properties of the surrounding interstellar medium. Beyond the physics of the bow shocks themselves the size and shape can be used to infer parameters of the stellar wind of the star as wel as the local interstellar medium. We propose to use both the PACS and SPIRE instruments in photometric mode to charaterise in detail the far-infrared spatial structure and corresponding spectral energy distribution of these bow shocks. In particular, we expect to resolve, if present, turbulent instabilities.

Lead Scientist: Nick Cox

Allocated time: 4.7 hours

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Green Light from AGB StarS (GLASS)

Ejecta of low to intermediate mass stars dominate the total mass that is re-injected by stars into the interstellar medium (ISM). Nevertheless, the mechanisms at the origin of the mass-loss of these stars are still not completely understood and their mass-loss history is still unknown. The problem is especially acute for the oxygen-rich evolved stars, for which theoretical models can still not predict the acceleration of the observed stellar winds.

During the last decade, new evidence arose from various wavelength domains that the mass-loss is varying in time, and is spatially structured, rather than constant and isotropic. In particular, Decin et al. (2011) analyse the structures revealed by Herschel-PACS around CW Leo, up to distances about five times larger than any structures that could be revealed with the VLT (FORS1). They show that the PACS "green" band (100 microns) is critically important to detect the complex density structure in the circumstellar material around CW Leo.

Detailed studies of the exact structure of the circumstellar material is of course crucial to set constraints on the mechanisms at the onset of mass-loss. Consequently, we propose to observe five objects at 100 microns with Herschel-PACS. Four of these objects are oxygen-rich, and all five have already revealed complex circumstellar structures at 70 or 160 microns, or in the sub-mm wavelength domain.

Lead Scientist: Pierre Royer

Allocated time: 4.5 hours

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Unveiling the evolutionary paths of the most massive stars through the study of their ejected nebulae

Several important questions remain open regarding the latest stages of evolution of the most massive stars, in particular regarding the exact evolutionary paths between the various subtypes of O stars, LBVs and Wolf-Rayet stars, and the mass-loss history of these objects throughout their lives.

In the framework of the MESS and MESS2 guaranteed projects, we have obtained or will obtain PACS imaging of 9 massive star nebulae of various types and PACS spectroscopy of 4 of them. In this short follow-up proposal we want to obtain PACS line spectroscopy for 3 peculiar massive and evolved objects for which spectroscopy is lacking. In particular, these observations will allow to determine the elemental abundances in the nebulae as well as the mass of the neutral gas using the fine structure lines formed in the ionized gas and in the photo-dissociation region respectively.

Lead Scientist: Pierre Royer

Allocated time: 3.7 hours

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Additional molecular observations in AGB and post-AGB objects

The study of shells around AGB and post-AGB stars with HIFI is being very fruitful. Observations of molecular lines have yielded extremely rich information on these objects, particularly on their warm gas components, which are crucial to understand the nature of the nebulae and the late stellar evolution.

Two guaranteed time projects, HIFISTARS and SUCCESS, have already obtained most of the originally proposed observations, yielding several hundreds of high-quality line profiles. But these results also indicate that some interesting observations were not proposed, because we could not imagine some unexpected results or because our samples were biased to previously well observed objects. We have realized that a small additional amount of time would significantly improve the existing data in three aspects: A) A few interesting O-rich AGB stars in the HIFISTARS sample were not observed in the strongest lines of H218O and H217O, which are in general found to be more intense than expected. B) We detected surprisingly strong and widespread NH3 J,K=1,0-0,0 emission, but a proper analysis of these lines obviously requires data on higher-J lines. C) Our sample of post-AGB objects was biased towards northern sources, the best studied ones until now, but the proper exploitation of ALMA will require high-frequency data of southern nebulae.

We are make observations of evolved stars that, in combination with already existing data, will significantly improve the reach and quality of the previous projects.

Lead Scientist: Valentin Bujarrabal

Allocated time: 12.5 hours

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Filler Projects

The brightest evolved stars in our Galaxy

To use the Herschel observation ime as efficiently as possible, very short observations are required.  This filler programme observes the brightest old stars in our Galaxy.  This includes making observations of several hundred stars with a range of chemistries and ages, using all three of Herschel's instruments.

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Open Time (Round 1)

Search for fallback disks around nearby, slowly rotating neutron stars

We propose to observe eight nearby, slowly rotating, strongly magnetized neutron stars with the Herschel PACS detector to search for dusty fallback disks and infer their properties. Such disks are generally predicted to be created after the supernova explosion by ejecta that fail to escape and remain bound. However, despite several searches, neutron star fallback disks remain elusive. Only one possible fallback disk has been detected so far -- around the slowly rotating (P = 8.7 s) magnetar 4U 0142+61 at a distance of around 4 kpc. Our proposed target sample includes seven radio-quiet, middle-aged neutron stars with similar periods, P = 3 - 12 s, and strong magnetic fields, B = (1 - 3) x 10^(13) G (the so-called Magnificent Seven), and one radio pulsar with similar P and B. All targets are much closer than 4U 0142+61 (d < 1 kpc). Their slow rotations allow the fallback disks to survive and increase the chance for disk detections. Our targets belong to the class of neutron stars that is discussed as a link between the ordinary rotation-powered pulsars and the magnetars. The possession of a fallback disk is one of the key parameters for understanding these different populations.

Lead Scientist: Bettina Posselt

Allocated time: 15.5 hours

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Dust processing and grain growht in Keplerian discs around binary post-AGB stars.

We propose to use the Herschel-PACS/SPIRE instruments to study the far-infrared continuum emission of a sample of evolved stars, both in the Galaxy and the Large Magellanic Cloud (LMC). All these stars are proven or suspected binaries surrounded by a long-lived circumbinary disc. Our analysis of the SPITZER mid-infrared spectra of these stars showed that grain processing is very effcient in these discs, despite the very short evolutionary timescale of the post-AGB central star. Extending the spectral energy distribution (SED) to far-infrared wavelengths is an essential complement in constraining the disc characteristics. The long-wavelength flux is very sensitive to the amount of large grains, and thus the total dust mass, and grain-size distribution within the disc. The ultimate goal of our research is to study the structure, formation and evolution of the very common discs around evolved binary stars, and constrain their impact on the evolution of the binary systems.

Lead Scientist: Clio Gielen

Allocated time: 15 hours

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Far-IR bands of PAHs in the Red Rectangle

Observations in the far-IR window should make a major contribution to solving two major problems in molecular astrophysics - the identity of the carriers of the Aromatic Infrared emission Bands (AIBs) and the Diffuse Interstellar absorption Bands (DIBs). The DIBs are generally attributed to carbon-based molecules but none has been assigned. Polycyclic aromatic hydrocarbons (PAHs) are commonly accepted to be the carriers of the AIBs but the hypothesis suffers from the lack of identification of individual species. This seriously limits the potential of these spectral signatures as probes of astrophysical conditions and processes. We propose to exploit the unique capabilities of Herschel to record the far-IR emission features of PAHs in the Red Rectangle. These features are very specific to the exact molecular identity and are a very attractive route for the spectroscopic identification of PAHs. Most of them carry a sharp Q branch increasing the contrast for their detection. We ask for 24.8 hours to perform deep PACS and SPIRE FTS spectroscopy to search for these Q branches. The unique carbon-rich Red Rectangle nebula is in an active stage of dust condensation and displays the strongest AIBs known as well as emission bands connected to the DIBs. Small molecules are underabundant and the Red Rectangle is clearly the place where the formation of large molecules can be tracked and the link between the AIB and DIB carriers should be explored. The analysis of the bands will be performed using spectroscopic data, both theoretical and experimental, as well as Monte Carlo modelling to simulate the emission process. Even in the worst case of no band identification, comparison of model predictions with the Herschel spectra will strongly constrain the PAH model. If some lines are identified, we will apply for follow-up observations with HIFI to resolve the hot band structure of the Q branches and structure in the P and R branches. Considering the short life of Herschel, this proposal is therefore very timely.

Lead Scientist: Christine Joblin

Allocated time: 24.8 hours

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Molecular complexity of the O-rich pre-Planetary Nebula OH231.8

Envelopes around evolved stars are extraordinarily efficient factories of complex molecules and dust particles that will eventually enrich the ISM. To date, most chemistry studies have focused on C-rich envelopes, expected to show the largest molecular variety and abundances. Recent observations suggest that O-rich shells may be as chemically diverse as their Carbon counterparts, however, no serious attempt has been made yet to perform a full frequency scan of such objects. We propose to carry out a line survey with HIFI of the well known, O-rich pre-PN OH 231.8, which displays fast (>400 km/s), bipolar outflows and a molecular richness unparalleled amongst O-rich AGB and post-AGB stars. Results from our exploratory mm line survey with the IRAM 30 m antenna of this object has led to the detection of hundreds of lines from different species that clearly point to non-equilibrium/shock-induced molecule formation processes in the fast flow. The HIFI could be potentially effective to reveal the dense and warm regions near the centre, which is strongly affected by shocks. At its completion, this project will provide the sorely needed molecular inventory and abundance measurements in O-rich CSEs, which are required to obtain a better understanding of the intricate, non-equilibrium molecule formation processes in O-rich environments. We will use our radiative transfer and chemistry models to reproduce the observed spectra and empirically derived abundances, respectively, and constrain the nebular physical and chemical conditions in the envelope of OH231.

Lead Scientist: Carmen Sanchez Contreras

Allocated time: 32.4 hours

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The puzzle of water vapour in carbon-rich stars

Using the HIFI instrument, we will address the puzzling - but widespread - appearance of water vapour in carbon-rich stars. Following up on detections of water in ALL SIX carbon-rich AGB stars observed to date in a pilot study performed in the HIFISTARS Key Program, we will target additional water transitions in four stars already observed or expected to show the most luminous water emissions. The target stars are CIT6, IRAC 15194-5155, V Cygni, and S Cep, and the additional transitions are the 4(22)-3(31) and 3(12) - 2(21) transitions at 916 GHz and 1153 GHz. Combined with spectra already obtained for the low-lying water transitions, and interpreted in the context of water excitation models, the proposed observations will place strong constraints upon the location of the emitting water. We will therefore be able to distinguish between various hypotheses that have been proposed for the origin of the observed water: the vaporization of orbiting comets or dwarf planets; catalytic formation on dust grains; or chemical processes initiated by the photodissociation of CO. In addition, we will carry out deep integrations to observe the lowest 1(11) - 0(00) transition of para-water at 1113 GHz in two carbon-rich AGB stars: IRAS+40540 and V Hya; here, ortho-water has been securely detected but existing observations of the 1113 GHz para-water line yield weak detections that lack the signal-to-noise ratio needed to constrain the ortho-to-para ratio.

Lead Scientist: David Neufeld

Allocated time: 28 hours

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A detailed study of the physics and chemistry in the Planetary Nebula NGC7027

Molecular spectroscopy at sub-mm and FIR wavelengths is an extremely powerful tool to investigate the latest stages of stellar evolution. The process of mass loss from evolved stars is not well understood, and molecular emission offers a unique avenue to trace the kinematics and structures of such objects, from AGB stars and their successors, Planetary Nebulae (PN), to massive supergiants. In addition, circumstellar shells of evolved stars foster a remarkable chemistry, producing unusual molecular species not easily observed in other environments, including long carbon chains, anions, metal-bearing molecules, and phosphorus compounds. The mechanisms by which this chemical synthesis takes place, its relationship to dust formation, and its evolution during the AGB and post AGB-phases, are all important questions that have yet to be understood. This proposal and the related other proposals submitted by our collaborators seek to answer these questions. Through the study of the emission from specific species combined with selected spectral surveys towards the PN NGC7027, we aim at understanding more specifically the physics taking place in the beginning of the PN stage, more exactly to precise the physical conditions ruling the molecular envelope, the ionized region and the thin interface between both, i.e. the PDR, through the quantification of the UV and shocks influence.

Lead Scientist: Fabrice Herpin

Allocated time: 19.4 hours

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Clues to the Evolutionary History of the R Coronae Borealis Stars

The unexpected discovery that the R Coronae Borealis (RCB) stars have isotopic abundances of 18O which are up to 500 times greater than those seen in other stars has increased the likelihood that these rare hydrogen-deficient carbon stars are the result of a double degenerate white dwarf (WD) merger (DD) rather than a final helium shell flash (FF). But other pieces of the puzzle have yet to be explained. For instance, a few RCB stars, including R CrB itself, show enhanced Li abundances, which favors the FF scenario. Also, IRAS images show that several RCB stars possess large dust shells which are consistent with fossil Planetary Nebula (PN) shells where the gas is no longer ionized. If these are PN shells, then that would point strongly toward the FF scenario. But other explanations for the dust shells include the FF outburst itself and new dust from the recent RCB phase. In addition, newly acquired Herschel/Spire images have called into question the reality of the shells themselves. The much improved spatial resolution over previous IRAS and MIPS images shows that some or all of the R CrB 100-micron shell is due to a cluster of galaxies lying directly behind the star. It is imperative that new high spatial resolution PACS and Spire images be obtained to verify the existence of these RCB star dust shells, as well as to analyze the morphology of the shells to deduce their nature and help decide between the DD and the FF scenarios. Furthermore, the new Herschel images, covering the wavelength range 70-500 microns, will be used together with archival IRAS, Spitzer and other datasets to produce SEDs for each star which will be modeled using Monte Carlo radiative transfer codes to determine the total dust mass, and total mass loss over the lifetime of the stars. It is a very exciting prospect that the RCB stars could be shown to be the product of a WD merger. This small group of stars would become even more interesting as they would be low-mass analogies to Type Ia supernovae.

Lead Scientist: Geoffrey Clayton

Allocated time: 25.6 hours

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How Cool Are Planetary Nebulae?

We propose to observe the [O III] 52 and 88 micron fine-structure lines with PACS in planetary nebulae, in order to investigate whether they contain a cold and possibly metal-rich component in addition to the ordinary hot (10,000 K) material. The presence of cold gas has been proposed in order to account for the excessive strengths of optical emission lines from recombining O++ ions, and low temperatures (500 - 5000 K) indicated by some diagnostics. If planetary nebulae truly have such inhomogeneous physical conditions and abundances, this calls into question our fundamental understanding of the composition of the nebular gas, a major source of recycled material to the ISM. The FIR [O III] lines offer a unique opportunity to address this issue because, due to their low excitation energies, they are emitted from both cold and hot gas, while the optical [O III] lines arise only from hot gas. The bright 52 and 88 micron lines were easily observed in some of our sample using less sensitive, large-beam instruments such as the Kuiper Airborne Observatory (by this P.I.) and ISO-LWS. For several targets the previous observations indicate O/H values greater than solar, supporting the idea that metal-rich gas is present, but the higher angular resolution of PACS is needed in order to discern whether the metal-rich material is concentrated towards the central regions, as claimed by optical studies. For several objects we propose to also measure the 88.8 micron 13–12 H I line, which will not only provide a measurement of H+, but also a probe of temperature by comparing its strength to that of shorter-wavelength H I lines. The latter will be taken from Spitzer-IRS maps (for a few targets), and optical integrated-field unit spectral maps either already obtained, or to be obtained, at McDonald Observatory. These observations may enable us to prove or disprove the presence of cold O-rich material in planetary nebulae.

Lead Scientist: Harriet Dinerstein

Allocated time: 6.1 hours

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Additional Hpoint observations of large post-AGB sources from HIFIStars

One of the most spectacular phases in the evolution of intermediate mass stars takes place at the end of their lives. At the end of the AGB, the central star dashes across the HR diagram from the red giant to the blue dwarf region. At the same time, the spherically symmetric and slowly expanding circumstellar envelopes around AGB stars become planetary nebulae (PNe), displaying a large variety of shapes and structures far more complex. This transformation takes place at the very end of the AGB, and it is due to the interaction of fast and bipolar molecular winds with the fossil AGB circumstellar envelope. The origin of these post-AGB winds is still unclear, but we know that the resulting two-wind interactions are only active during a very short period of time, ~ 100 yr, but still they are able to strongly modify the kinematics of the nebulae and re-shape them. To better understand these processes we must study the warm molecular gas component of early post-AGB sources, both pre-planetary nebulae (pPNe) and young PNe. Herschel/HIFI is very well suited at this, because its spectral coverage, high velocity resolution, and superb sensitivity. For these reasons, 10 pPNe and young PNe were included in the KPGT HIFIStars, were a large number of spectral lines are observed in a moderate number of frequency setups, but just at the central point. In many cases this is simply enough, since most of post-AGB sources in HIFIStars are compact. However there are three cases in which the non spherically symmetric structures seen in the warm gas are larger than the beam of the telescope: OH 231.8+4.2, NGC 7027 and NGC 6302. Therefore we propose to perform some additional pointing in these three sources in a selected sample of HIFIStars frequency setups, were we have detected strong lines of CO, H2O, NH3 and OH. These observations are crucial to understand the kinematics and interactions traced by these warm gas probes, and so gain insight in the intricate problem of the post-AGB dynamics.

Lead Scientist: Javier Alcolea

Allocated time: 13 hours

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Characterising the Environment of Fullerene Formation

The bulk of the dust that is injected into the interstellar medium is formed via a complex chemistry in AGB circumstellar environments, which transforms the atomic gas into molecules and dust. To date, more than 60 individual molecular species of both inorganic and organic nature and a handful of dust minerals have been identified in these outflows. These environments are also thought to be the birthplace for large aromatic species such as polycyclic aromatic hydrocarbons and fullerenes. Because of their remarkable stability, fullerenes have been predicted to survive the harsh conditions of the insterstellar medium. However, targeted searches for the presence of fullerenes in various astrophysical environments were unsuccessful or not conclusive, until now. The Spitzer mid-IR spectrum of Tc1, a young planetary nebula with a low excitation central star and a high C/O abundance ratio, shows clear traces of C60 and C70. This indicates that when conditions are favorable, fullerenes are formed in large quantities. The mid-IR spectra allow us to quantify the abundance of carbon in the fullerenes, estimate their temperature, and detect them in the solid state in circumstellar environments, but to date we have little information on the circumstellar dust or the composition of the gas. We propose to use the far-IR spectrum of Tc1 using the Herschel PACS spectrometer to study the energy balance and characterize the conditions in the circumstellar regions where fullerenes are formed.

Lead Scientist: Jeronimo Bernard-Salas

Allocated time: 6 hours

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Molecular complexity in the circumstellar envelope of the O-rich evolved star VY CMa

Envelopes around evolved stars are factories of complex molecules (neutrals, anions and cations) and one of the main sites for dust formation. Most chemical studies have focused on C-rich envelopes, in particular on IRC+10216. Recent observations of metal bearing species and hydroxides in O-rich shells suggest that the chemistry in these objects can be as richer as in C-rich circumstellar envelopes. We propose to perform a complete line survey of the prominent O-rich star VY CMa with HIFI. This study will provide, in addition to all lines of CO, H2O, HCN, SiO, SO2, a complete inventory of the molecular gas in O-rich evolved stars. The comparison with the line survey of IRC+10216 (already carried out with HIFI/Herschel) will permit to distingush the different chemical processes in these objects and the subtle role of the C/O abundance ratio in the molecular composition of the gas at different distances from the central star. We will use our radiative transfer and chemistry models to reproduce the observed spectra and to derive molecular abundances. The data will also allow a better understanding of the physical structure of the envelope.

Lead Scientist: Jose Cernicharo

Allocated time: 54 hours

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Shock Chemistry and Dust Processing in Interacting Supernova Remnants

Supernova remnants interacting with dense moelcular clouds provide astroc-hemical laboratories to study heating and cooling of the dense ISM, shock chemistry, destruction and sputtering of dust, and the reformation of molecules. Water is expected to be a major coolant for shocks into dense gas, yet the number of remnants in which IR lines of hydroxyl and water are detected is very limited. We propose Herschel PACS, SPIRE and HIFI observations of three remnants with particularly high shocked gas densities, high dust and IR line luinosities, and extreme ionization environments. The scientific objectives of this proposal are: (1) to determine the abundance and excitation of oxygen-bearing molecules, (2) to study the oxygen chemistry in dense molecular gas shocked by powerful supernova remnant blast waves, and (3) to directly measure dust processing and constrain the effects of dust on shocks and oxygen chemistry.

Lead Scientist: John Hewitt

Allocated time: 18 hours

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A HIFI survery of Water Fountain stars: unveiling the inner structure of hit star envelopes.

``Water fountain'' stars (WF) are considered key objects to our knowledge of the sun-like star evolution processed. At some point soon after AGBs, WFs would develop very fast and highly-collimated outflows, which produces H2O maser emission, spread over more than 100\,\kms, when hitting the AGB previous envelope. Very recently, our group has detected low-J CO and 13CO line emission arising in the cold, outer parts of the envelope. Based on these results, we ask for HIFI observations of mid- and high-J lines of CO, together with a set of thermal H2O lines, and CI. With these data, we plan to probe the inner, hotter, and denser parts of the envelopes, and to give some insights about the mass-loss history and chemistry; possibles PDRs will also be tested. This information will provide valuable inputs to our models, such as the total mass, and temperature profile. Some physical parameters will be also constrained. We have selected the whole sample of known WFs, a total of 14 sources (two of them still unpublished), for this project.

Lead Scientist: Jose Ricardo Rizzo

Allocated time: 3 hours

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Measuring the electron spectrum generated by diffuse shock acceleration in the Pulsar Wind Nebula HESS J1632-478.

Relativistic collisonless shocks propagating in plasmas are an essential ingredient to explain particle accelerators such as gamma-ray bursts, supernovae remnants, pulsar and stellar winds. Numerical simulations predict particle energy distributions but these are very difficult to test because of the lack of simultaneous observation over 20 decades of frequency, complex geometries or variability. We propose to use SPIRE in the Small Map configuration to map the infrared emission of a very energetic and old pulsar wind nebula recently detected by HESS. Exploiting the unique resolution and sensitivity of SPIRE at 250, 350 and 500 um, we aim at detecting the faint synchrotron emission that will be compared to existing measurements in the radio, X-rays and gamma-rays. These wavebands, located between the CMB and dust emission, are centered where the synchrotron emission from the predicted relativistic Maxwellian distribution peaks, providing a constraining flux measurement.

Lead Scientist: Matteo Balbo

Allocated time: 0.5 hours

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Herschel PACS and SPIRE investigation of water fountains

Water fountains (WFs) are late AGB and post-AGB objects that show the earliest known manifestation of collimated mass-loss in evolved stars, with jets that have dynamical ages of only 5-100 yr and sizes as small as 100-500 AU. These fast collimated outflows, impinging onto the circumstellar material at the end of the AGB phase, are presumably responsible for the shaping of asymmetric planetary nebulae (PN), with the highly asymmetric H2O-maser-emitting PNe being presumably the immediate descendants of WFs. Therefore, WFs and H2O-PNe are key objects to study the shaping of PNe. Since their thick envelopes make them strongly obscured in the optical and near-IR, their inner circumstellar structure would be best traced at longer wavelengths. Here we propose the acquisition of Herschel PACS and SPIRE deep imaging photometric observations of all known WFs and H2O-PNe to be added to near-IR, mid-IR, submm, and mm data to continue building the spectral energy distribution (SED) of these sources, and to check for the presence of extended material. The characterization and theoretical modeling of their SEDs will be used to infer the presence of disks and to estimate the physical parameters of circumstellar disks and envelopes, which in their turn will impose constraints on the models for the generation of jets in evolved objects.

Lead Scientist: Martin A. Guerrero

Allocated time: 17 hours

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Herschel SPIRE FTS observations of mass loss from a red supergiant in the Large Magellanic Cloud

We propose to observe submillimeter CO thermal lines in the brightest red supergiant (RSG) in the Large Magellanic Cloud (LMC). We will use Herschel/SPIRE in the FTS mode. The high sensitivity of this instrument enables us to detect CO thermal lines in a RSG beyond the Milky Way for the first time. The objectives of this programme are (1) to obtain the gas mass-loss rate from the RSG, (2) to evaluate the gas-to-dust mass ratio of the RSG, and (3) compare the gas-to-dust mass ratio of the RSG with that of the LMC interstellar medium (ISM). Those will place important constraints on whether RSGs are important contributors to the chemical enrichment of the ISM. RSGs lose a large quantity of mass through stellar winds, and these winds consist mainly of molecules whose mass can be measured by the CO thermal lines. Using the newly obtained gas mass-loss rate, and the previously obtained dust mass-loss rate, we will estimate the gas-to-dust mass ratio of the circumstellar envelope (CSE) of the RSG. Of particular interest is whether the gas-to-dust mass ratio is affected by the metallicity of galaxies. The metallicity of the LMC is about half of the solar metallicity, and we will compare our measurements with those of Galactic RSGs. Dust grains are composed of metals, and we expect a higher gas-to-dust mass ratio in the LMC. Further, we will compare our measured CSE gas-to-dust mass ratio to the LMC ISM value. That would aid determining whether these dying stars are important sources of gas and dust in the ISM, or if dust grains gain mass from the gas phase in the ISM, using the dust injected by dying stars as seeds. If the dust mass increases in the ISM, the gas-to-dust ratios should differ between the CSE and the ISM. This will provide the first direct observational evidence of whether additional dust depletion is important in the ISM. This small (7.7 hour) project will have a great impact on our understanding of both stellar physics and ISM evolution.

Lead Scientist: Mikako Matsuura

Allocated time: 7.7 hours

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Clumping in OB-star winds

Massive stars, their nature and evolution, play a important role at all stages of the Universe. Through their radiatively driven winds they influence on the dynamics and energetics of the interstellar medium. The winds of OB stars are the most studied case. Commonly, the mass-loss rates of luminous OB stars are inferred from several types of measurements, the strengths of UV P Cygni lines, H-alpha emission and radio and FIR continuum emission. Recent evidence indicates that currently accepted mass-loss rates may need to be revised downwards when small-scale density inhomogeneities (clumping) are taken into account. We argue that only a consistent treatment of ALL possible diagnostics, scanning different parts of the winds, and analyzed by means of ‘state of the art’ model atmospheres, will permit the determination of true mass-loss rates. To this end we have assembled a variety of multi-wavelength data, but one crucial observational set is missing: far-IR diagnostics of free-free emission, which uniquely constrain the clumping properties of the wind at intermediate heights. We propose, therefore, to use PACS photometric mode to fill this crucial gap, studying the 70 and 110 micron fluxes of a carefully selected sample of 29 O4-B8 stars. These observations will provide the missing information to derive the clumping properties of the entire outflow, to understand the wind physics, and to obtain reliable mass−loss rates.

Lead Scientist: Maria del Mar Rubio

Allocated time: 9.4 hours

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Hunting for missing evolved stars in the Galactic plane

We discovered more than 400 compact shells in the MIPSGAL 24 microns survey of the Galactic plane. While only 10% of them are detected at shorter wavelengths, about a third are visible in the MIPSGAL 70 microns survey. About 15% of all the objects have been identified previously as planetary nebulae, supernova remnants, Wolf-Rayet stars, luminous blue variables. Spectroscopic follow-ups on a limited sample in the near-IR and mid-IR have revealed several dust-free planetary nebulae with very hot central white dwarf as well as a significant increase in the number of WR/LBV candidates. The remaining 350 or so unknown bubbles are also expected to be evolved stages of low- to high-mass stars that could account for the "missing" evolved stars in the Galaxy. To determine the true nature of a significant fraction of the unidentified bubbles, we propose to observe 35 of the brightest objects in the MIPSGAL 70 microns images with PACS Range Spectroscopy. With these data we will constrain the origin of their far-IR emission. Comparing the proposed spectroscopic to those of known evolved stars from the MESS Guaranteed Time Key Program will allow us to unequivocally identify the MIPSGAL bubbles. The richness of the far-IR emission spectrum of these evolved stellar objects will also provide us with dust and gas (atomic and molecular) features, with which we will characterize in details the physical conditions within each bubble thanks to modeling of these features. "Unveiling hidden details of star and galaxy formation and evolution" is the Herschel observatory's statement, as the mission brochure's cover shows. The observations that we propose here are in perfect resonance with that statement as we aim at lifting the veil on evolved stars hidden in the Galaxy. Furthermore, we propose observations that will build a synergy between two Key Programs (HiGAL and MESS).

Lead Scientist: Nicolas Flagey

Allocated time: 39.6 hours

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Oxygen abundances in carbon-type Wolf-Rayet stars from PACS scan spectroscopy

We seek 13.6 hours of spectral scans of the [OIII] 88 micron fine-structure line with PACS for a sample of Milky Way carbon and oxygen sequence Wolf-Rayet stars. The proposed observations will: (i) enable reliable oxygen abundances to be determined for WC stars for the first time, to test evolutionary predictions; (ii) refine the degre of clumpig in the outer stellar winds of these stars derived from existing ISO/SWS or Spitzer/IRS datasets.

Lead Scientist: Paul Crowther

Allocated time: 8 hours

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Physical properties of the dusty circumstellar envelopes of two bright classical Cepheids

The recent discovery of circumstellar envelopes (CSEs) around Cepheids is an indication that many Cepheids, if not all, are surrounded by CSEs. The bright classical Cepheids RS Pup and delta Cephei are particularly interesting members of their famous class of variable stars, as they are known to be surrounded by large CSEs that can be resolved angularly by Herschel. The requested observations will probe the cold extensions of these nebulae, and will be the first observations of any Cepheid in this wavelength domain. The present proposal aims at characterizing the dust in the CSEs using Herschel/PACS and SPIRE imaging photometry. Our scientific goal is to understand the origin of the envelopes: were they formed through evolutionary mass-loss from the Cepheids or are they made of residual interstellar material pre-existing the formation of the Cepheid progenitor? This question is of critical importance both to understand the evolution of the Cepheids themselves (stellar physics) and for the future application of the period-luminosity relation in the thermal-IR domain with the James Webb Space Telescope (extragalactic distance scale). The formation mechanism of the large dusty nebula surrounding these Cepheids is currently unknown, but two hypotheses appear plausible: (A} - it is a result of evolutionary mass-loss from the star through stellar wind, possibly linked to pulsation, that condensed into dust at large distances from the star. (B) - it is a remnant of the interstellar medium from which RS\,Pup formed, blown away by the stellar wind from the Cepheid. Our goal is to test these two hypotheses and characterize physically the cold dust in the envelopes (mass, temperature, distribution, composition).

Lead Scientist: Pierre Kervella

Allocated time: 2.8 hours

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Herschel's Opportunity to Solve the Nebular Abundance Problem While Creating a Legacy Planetary Nebulae Dataset

Abundance surveys of a large sample of Galactic planetary nebulae (PNe) have led to the discovery of a group of super-metal-rich nebulae whose spectra show prominent optical recombination lines (ORLs) from C, N, O, and Ne ions. The heavy element abundances derived from ORLs for several PNe are a factor >10 higher than those derived from the traditional method based on collisionally excited lines (CELs). This ratio is called the abundance discrepancy factor (adf). A promising proposition to explain the nebular abundance problem posits that these nebulae contain (at least) two distinct regions - one of "normal" electron temperature, Te (~10000 K) and chemical composition (~solar) and another of very low Te (< 1000) that is H-deficient, thus having high metal abundances relative to H. The latter component emits strong heavy element ORLs and IR fine-structure (FS) CELs but essentially no optical/UV CELs. Efforts to directly detect these inclusions in PNe have been unsuccessful to date. However, there is mounting circumstantial evidence for their existence, such as presented in our recent paper that modeled the high-adf PN NGC 6153 using a 3-D photoionization code. The models that included the low Te, H-deficient knots fit most observations far better than did those models without the clumps. With the launch of Herschel, there is finally the capability to perform a test we've been dreaming of. Measurements have shown that the adf varies with position in a PN and is highest close to the central star. The very low Te inclusions must be cooled via FS IR lines. We propose to use Herschel to map the FS IR lines in 5 bright PNe on the largest adf list, to find if these lines peak where the adf peaks. These spectra will also provide a feast for our other team expertise/interests: a legacy dataset of molecular lines to explore PDRs, how the central star interacts with the AGB ejecta and shapes the PN, how the shocks are produced, what comprises the chemistry of the molecular ejecta, and how do PNe evolve.

Lead Scientist: Robert Rubin

Allocated time: 33.3 hours

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A Statistical Sample of Planetary Nebulae in the Galactic Bulge: Measuring Masses and Mass-Loss Rates

We propose to exploit Herschel's unprecedented sensitivity to obtain 100-500 micron photometry for a statistical, flux-limited (in the IRAS 60 micron band) sample of Galactic bulge planetary nebulae (GBPNe), using PACS and SPIRE. The Galactic Bulge volume, offers a unique, nearby environment where a statistical population of PNe, all at roughly the same well-established distance, can be studied in order to understand these objects, test theoretical models for their formation and evolution, and address the mystery of the constancy of the PN luminosity function (an important cosmological distance indicator). Much of the mass ejected during the preceding AGB phase is expected to lie outside the ionised shells in these objects, and can only be detected via the thermal emission from cold dust. Herschel provides us the only platform to measure the mass budgets of a statistical sample of GBPNe, and thus help us address one of the longest standing astrophysical problem: the relationship between the birth mass of solar mass stars and the mass left at the end when they die. PNe are an important contributor to the total mass return to the ISM for the old Bulge population, and the proposed observations will allow us to infer the PN contribution to the total rate of mass loss in the Bulge, a crucial input to evolutionary models. The robust constraints to the progenitor masses of PNe from our study will allow elemental enrichments to be determined as a function of initial stellar mass, providing key information for models of AGB nucleosynthesis. The proposed observations are a critical component of a broader study comprising existing and future ground- and space-based observations that will produce a combination of nebular masses, mass-loss rates, luminosities, physical sizes, morphologies, radial velocities, ages, and chemical abundances for a statistical sample of planetary nebulae, resulting in a unique dataset describing the final stages of stellar evolution in unprecedented detail.

Lead Scientist: Raghvendra Sahai

Allocated time: 25 hours

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Searching for Interrupted Mass-Loss in AGB stars: A Herschel Survey

Although the massive winds of AGB stars are reasonably well understood as resulting from radiation pressure on dust grains, our knowledge of how the mass-loss rates change as stars ascend the AGB is very poor. A small number of carbon-rich AGB stars have been found with detached circumstellar shells that imply that their mass-loss rates dropped dramatically a few thousand years ago. This decrease has been hypothesised to result from a He-shell flash which is believed to occur periodically in these stars; the nucleosynthesis of carbon in this flash, and its subsequent dredge-up to the stellar surface, converts oxygen-rich stars to carbon-rich ones. We propose to use Herschel's unprecedented far-IR sensitivity and angular resolution to make a systematic search for signatures of interrupted mass-loss such as detached shells in a list of 21 targets which includes C-rich, O-rich and S-type stars (C/O~1) to test this hypothesis. Our list has been constructed using the IRAS point-source catalog to identify objects which have "60-micron excesses": their 60 to 25 micron flux ratio is > 0.4 and thus significantly larger than the average ratio for AGB stars. This excess implies the presence of a cold, extended dust shell, and relatively little hot dust close to the star, as compared to the average mass-losing AGB star. The morphology of detached shells provides a clear indication as to its origin (circular rings due to interrupted mass-loss, paraboloidal bow-shock shaoes due to interaction of the AGB wind with the ISM): hence our proposed PACS observations will thus be able to distinguish between detached shells due to interrupted mass-loss and ISM interactions.

Lead Scientist: Raghvendra Sahai

Allocated time: 13.5 hours

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Herschel/HIFI observations of water fountain sources

One of the most intriguing aspects of the evolution of intermediate mass stars is how these simply spherical balls of gas can evolve resulting in the formation of PNe, which display a variety of shapes far from being isotropic. This transformation takes place after the end of the AGB, when some post-AGB flows collide with the CSEs of the former AGB star. These envelopes, which are spherical and expanding at a low velocity, are accelerated and re-shaped due to the interaction of the post-AGB flows, which are much faster and highly bipolar. Later, the star becomes much hotter, ionizing the surrounding material and forming the PN. This wind interaction is active only during a very short times, ~ 100 yr, and to gain insight into the subject we need to study objects in which this transformation is taking place, or it has happened very recently. So far, the earliest post-AGB sources we know about are the "water fountains". These are OH/IR sources in which, contrarily to what happens in AGB stars, the H2O masers show very wide velocity ranges, wider than OH masers. VLBI observations tell us that in "water fountains" H2O masers trace very fast bipolar outflows, with kinematic ages as short as 40-100 yr, comparable to the expected duration of the post-AGB acceleration of the envelope. Unfortunately, the main properties of water fountain nebulae are now well known, as the masers do not provide information on the density and temperature of the gas. The usual probe, low-J transitions of CO, becomes unusable in most cases, as these sources suffer from strong interstellar contamination. We can overcome these problems with Herschel/HIFI, by observing higher J-transitions of CO, where contamination is negligible, and the strong lines of H2O and OH available within the band. Here we propose to observe six water fountains in five frequency setups, to study the molecular envelope of these sources by means of the observation of several lines of these very abundant molecules in their envelopes.

Lead Scientist: Rebeca Soria Ruiz

Allocated time: 17 hours

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The Herschel view on supergiant High Mass X-ray Binaries: revealing the most obscured high-energy source of our Galaxy

Among the high-energy binary sources, a new type of sources has been recently discovered by the high-energy observatory INTEGRAL. They are constituted of intrinsically highly obscured supergiant High Mass X-ray Binaries, of which IGR J16318-4848, a compact object orbiting around a supergiant B[e] star, seems to be the archetype. These sources have been partly unveiled by means of multi-wavelength X-ray, optical, near- and mid-infrared observations. However the fundamental questions about these sources, namely their formation, their evolution, and the nature of their environment, are still unsolved. After the successful multi-wavelength observations that we have performed on these intriguing sources, we propose here to get Herschel/PACS photometric observations, in order to detect the presence and characterize the nature of absorbing material (dust and/or cold gas) enshrouding the whole binary systems. This study will allow us to get a better understanding of the formation and evolution of such rare and short-living high mass binary systems in our Galaxy.

Lead Scientist: Sylvain Chaty

Allocated time: 16 hours

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Measurements of the Atomic Carbon Isotope Ratio in Evolved Stars

We propose to use Herschel-HIFI to observe the atomic carbon isotope ratios of evolved stars. The sample of circumstellar envelopes to be studied are being observed by the HIFISTARS program, where the oxygen isotope ratios will be measured via H2O. The proposed observations will allow us to determine the carbon isotope ratio in various types of stars (C-rich, O-rich, and S-type) as well as constrain chemical fractionation that may occur in the outer envelopes of these objects and/or provide insight into other nucleosythetic processes that may alter the CNO-isotopic abundances. The data in this program are complimentary and will employ the results obtained from the HIFISTARS program.

Lead Scientist: Stefanie Milam

Allocated time: 16 hours

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The Homunculus: Clues to Massive Ejection from the Most Massive Stars

Eta Carinae is a lynchpin between mass ejection by highly evolved massive stars and the enriched ISM. The Homunculus, a very dusty, neutral bipolar shell ejected in the 1840s, is known to contain at least 12 solar masses, based upon gas/dust=100. But this ejecta is very N-rich with C and O being 0.02 that of solar abundance. What dust formed and how much total mass was ejected? Our ultimate goal is to obtain the total ejected mass. We propose to obtain full spectral scans of the Homunculus with PACS and SPIRES and selected scans with HIFI. We will use these spectra to identify molecules and atomic species associated with this C- and O-depleted gas. While most of this depletion is due to CNO processing and conduction in stars > 60 solar masses, additional depletion is likely due to the first molecules and dust formed at high temperatures during the 1840s eruption. In line of sight we see overabundances of metals not ordinarily seen in the ISM: Sr, Sc, V. These metals have been trapped in atomic state due to limited O and C being available to form molecules. Yet dust has still formed. But what kind of dust? Through existing HST/STIS and VLT/UVES spectra combined with the proposed Herschel spectra and detailed modeling, we will gain much better insight on how molecules and dust can formed in depleted C,O conditions, and in turn provide an improved estimate of the total mass loss. Implications from this study apply to the first massive stars and the earliest dust in the Universe.

Lead Scientist: Theodore Gull

Allocated time: 24.3 hours

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Characterizing the Mid/Far-Infrared Excesses of Cataclysmic Variables

Spitzer and IRAS observations have shown that cataclysmic variables (CVs) are sources of mid/far-infrared emission. While the excesses detected by Spitzer have been attributed to circumbinary disks (CB), as currently envisioned, such disks could not be reponsible for the IRAS detections. If due to dust, the IRAS detections imply much more extended CB disks then previously proposed, or perhaps, dust shells ejected by ancient classical novae eruptions. Alternatively, the Spitzer and IRAS detections may be due to synchrotron emission from these objects. Recent Spitzer and radio observations have now confirmed that CVs are synchrotron sources. If most CVs are synchrotron sources, this would solve the mystery of the unexpectedly large IRAS detection rate for CVs. Both CB disks and synchrotron emission can provide additional sources of angular momentum loss. Currently, there is considerable debate in the CV community over whether the commonly invoked mechanisms for momentum loss are sufficient to explain CV evolution. If most CVs have CB disks or synchrotron jets, they could provide the additional angular momentum loss required to keep CVs contact binaries. We propose to conduct a small survey spanning the major subclasses of CVs. All eight targets have mid/far-infrared excesses. Our program requires 7.0 hr of Herschel time.

Lead Scientist: Thomas Harrison

Allocated time: 7 hours

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Imaging and Spectroscopy of the Infrared Shell Surrounding the Pulsar Wind Nebula G54.1+0.3

We propose to obtain Herchel PACS imaging and spectroscopy of G54.1+0.3, a young pulsar wind nebula (PWN) surrounded by a shell of supernova (SN) ejecta and freshly-formed dust. The dust and gas seem to be illuminated by the expanding PWN and the members of a stellar cluster in which the SN exploded, making them observable at infrared wavelengths. Our recent Spitzer study suggests that the shell contains 0.1 solar masses of dust, possibly the largest amount observed to date, and that this dust has not yet been processed by supernova remnant (SNR) shocks. The proposed observations will allow us to 1) determine the composition, temperature, mass, and spatial distribution of pristine SN dust that has not been altered by shocks, 2) search for a colder dust component to constrain the total amount of dust in the system, and 3) measure composition and velocity of the surrounding SN ejecta that will provide information on the SN progenitor and the evolution of the PWN. A Hersche study of this unique system promises to shed light on the outstanding questions regarding the nature and quantity of dust produced in SNe.

Lead Scientist: Tea Temim

Allocated time: 8.9 hours

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Far-IR Emission from Planetary Nebulae: Simultaneous Mapping and Spectral Probing of the Multi-Phased Dusty Gaseous System

We propose to undertake an ambitious Herschel large survey of planetary nebulae (PNs), mustering the full strengths of Herschel's broadband mapping, spectral mapping, and spatio-spectroscopic capabilities. Our proposed PN survey will exploit Herschel's unprecedented spatial-resolving power in the far-IR wavelengths to its fullest potential. We will perform (1) deep PACS/SPIRE broadband mapping to account for the coldest dust component of the nebulae in the target PNs and determine the spatial distribution of the dusty PN haloes, (2) exhaustive PACS/SPIRE line mapping in far-IR fine-structure and CO transition lines in two representative PNs to diagnose the energetics of the nebulae as a function of location in the nebulae, and (3) PACS/SPIRE spectral-energy-distribution spectroscopy at several positions in the target PNs to understand variations in the physical conditions as a function of location in the nebulae. This PN survey is distinguished from the existing MESS KPGT program by the extra dimension added by spectral mapping and spatio-spectroscopy that permit simultaneous probing of the gas and dust component in the target PNs. Through these investigations, we will consider the energetics of the entire gas-dust system as a function of location in the nebulae. Herschel will allow us to take this novel approach which has rarely been taken previously. The proposed Herschel survey will be conducted in collaboration with the recently-approved Chandra X-ray Observatory Large Project to furnish substantial PN data resources that would allow us - a community of PN astronomers - to tackle a multitude of unanswered issues in PN physics, from the shaping mechanisms of the nebulae to the energetics of the multi-phased gas-dust system surrounding the central white dwarf. The proposed Herschel survey and the approved Chandra survey, combined with the community assets from optical to mid-IR, will provide an extremely valuable and comprehensive compilation of PN resource that carries a significant Legacy value.

Lead Scientist: Toshiya Ueta

Allocated time: 197.5 hours

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Low-excitation atomic emission from young planetary nebulae

AGB stars evolve to form blue dwarfs in a very short time, about 1000 yr. Simultaneously, the circumstellar envelope around the AGB star, which is spherical and expands at moderate velocity, also evolves to form a planetary nebula (PN) around the dwarf, which usually shows axial symmetry and fast bipolar flows. This change is due to very strong shock interaction between the fossil AGB envelope and fast and collimated post-AGB jets. The nature of the gas also changes: the AGB shells are molecular and cool, 50 - 500 K, while gas in evolved PNe is fully ionized and hot, about 10000 K. This change is thought to proceed progressively during the intermediate phase of protoplanetary nebula (PPN), in which an intermediate phase of low- or intermediate-excitation atomic gas is expected to develop, due to photodissociation (in a PDR) or to gas heating in the shocks. This neutral or slightly ionized atomic gas can only be observed by means of fine-structure lines of abundant atoms. ISO detected a few nebulae in such lines, but the relatively poor sensitivity of that instrument and the lack of spectral resolution prevented any deep study of this component. Herschel/HIFI can significantly improve those data, in particular yielding accurate line profiles, from which we will be able to identify the emission from the different nebular components. New observations will allow us to study the properties of low-excitation atomic gas in PPNe and, in particular, to discern the origin the molecular dissociation, in a PDR or in shocks, from analysis of the fine-structure line profiles. We accordingly propose observations of the CII (158 microns), CI (609, 370 mic), and OI (63 mic, using PACS) lines in a sample 13 objects, mostly PPNe and young PNe, but also including a few red (super)giants and more evolved PNe for comparison.

Lead Scientist: Valentin Bujarrabal

Allocated time: 24 hours

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Open Time (Round 2)


Mapping the Ejecta Around Massive Stars at High Galactic Latitudes

Very massive stars are infrequently found in the unvierse. However, their influence is substantial in terms of ionization flux and total stellar wind luminosity in local star forming regions. They are also believed to be very important in the earliest star forming era of the universe where metallicities are low and more massive stars are prefentially formed. Evolved massive stars later become the sites of type II supernova explosions. However, our understanding of the evolution of very massive stars is relatively limited, and many evolutionary sequences are still possible. A fundamental set of properties for understanding the evolution of very massive is there historical mass-loss. This is, in effect, imprinted on their surroundings. Wolf-Rayet stars are believed to be the evolutionary end points of very massive main-sequence O stars before they become supernovae. Many are known to have ring nebulae of material, most frequently found from the detection of ionized shells. Probing these shells should provide better constraints as to the mass of materials (and evolution of these materials) lost during the heavy mass-loss phases that are believed to occur prior to the Wolf-Rayet phase of a star. But the lack of understanding of particularly cooler material in ejecta has made this task difficult. The use of higher resolution far-infrared sata from {\it Herschel} PACS and SPIRE data on large nebulae should enable the structures of cool materials to be finally determined and masses measured in such nebulae. In the proposed project, we investigate the environments of five Wolf-Rayet stars at high galactic latitude, which considerably reducing line of sight and confusion problems found in the plane of the galaxy. Most have evidence of molecular/neutral materials from prior measurements. Our intent is to measure the mass, indicate the extent of clumping and cool materials in the nebulae, plus searching for evidence of extended low-level emission structures and any evidence of multiple ejections.

Lead Scientist: Anthony Marston

Allocated time: 17.8 hours

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CO emission from a molecular knot in the Cas A supernova remnant

Supernovae play a key role in the dust budget of galaxies producing dust from freshly synthesized elements in their ejecta and processing dust in their strong shocks both in the local and in the early Universe. Key to understanding the importance of the dust formed in supernova ejecta for this budget is to quantify the effects of the reverse shock in supernova remnants in processing ejecta dust, particularly for the dense knots where newly formed dust is best protected (shock velocity scales inversely with the square root of the density). Observations of molecules provide an ideal tool to determine the characteristics of dense knots processed by the reverse shock. We propose to measure five high J pure rotational lines of CO from a knot in the supernova remnant Cas A using PACS in spectroscopy mode. Bright ro-vibrational emission from CO molecules has been discovered in the near- and mid-IR by Spitzer, Akari, and Palomar but the low spectral resolution, Akari, 4.6 micron spectrum has insufficient resolution to determine the characteristics of the emitting gas, except that the emission originates from warm (~2000K) and dense (>5x10^5 cm^-3) gas being processed by the reverse shock. The proposed PACS spectroscopy observations are designed to accurately determine the excitation of CO and thereby the density, temperature, and mass of emitting CO in this knot. These are key to understanding the preshock conditions and shock velocity for this knot. Dense knots in the supernova ejecta are the best place for dust and molecules formed in supernova ejecta to survive processing by the reverse shock because the shock velocity is much lower than in the interclump medium and hence we expect them to play a key role in seeding the ISM with supernova-produced dust. The results of these observations will form the basis for studies of the effects of the reverse shock in dense ejecta clumps using our dust processing shock models.

 

 

Lead Scientist: Alexander Tielens

Allocated time: 7.2 hours

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Herschel PACS Spectroscopy of Mass Loss from Red Supergiant Stars in the Large Magellanic Cloud

We propose to measure the thermal CO lines J = 14-13 and J = 15-14 in the circumstellar envelopes (CSEs) of 2 of the brightest red supergiant (RSG) stars at far-infrared wavelengths in the Large Magellanic Cloud (LMC) - WOH G64 and IRAS 05280-6910. CO lines are useful for measuring molecular gas, which dominates stellar winds from low-temperature stars, such as RSGs. The scientific objectives are as follows: (1) What gas mass-loss rates do LMC RSG stars experience? Mass lost from RSGs consists mainly of molecular gas, so measurements of the gas mass-loss rate are crucial. (2) Of particular interest is the influence of low metallicities, so we will compare the gas-to-dust ratios of evolved stars in the LMC (half of the solar metallicity) and those in the Galaxy. (3) Finally, our measurements will help us understand whether or not RSGs play a key role in the life-cycle of matter within galaxies (e.g., as dust and gas injectors).

Lead Scientist: Benjamin Sargent

Allocated time: 2.6 hours

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Dust and gas in Keplerian discs around binary post-AGB stars.

We propose to use the Herschel-PACS/SPIRE instruments to study the far-infrared spectral properties of a sample of bright evolved binary stars. All these stars are surrounded by a dusty circumbinary disc in a stable Keplerian orbit. Mid-infrared spectroscopic studies show that these discs are the ideal environment for strong dust processing, in the form of grain growth and crystallinisation. Detailed disc modelling shows that the discs are not only very dusty, but also that a non-negligible gas component must be present, to allow for the observed disc geometry. The spectroscopic capabilities of Herschel will allow to extend the spectroscopic information to the far-infrared, allowing the detection of previously unexplored or unexpected dust and gas features. This will provide invaluable information on the dust processing in the disc, and the relatively unstudied gas component.

Lead Scientist: Clio Gielen

Allocated time: 14.3 hours

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Dust processing and grain growth in Keplerian discs around binary post-AGB stars.

We propose to use the Herschel-PACS/SPIRE instruments to study the far-infrared continuum emission of a sample of evolved stars. All these stars are proven or suspected post-AGB binaries surrounded by a long-lived circumbinary disc. Our analysis of SPITZER mid-infrared spectra of these stars showed that grain processing is very efficient in these discs, despite the very short evolutionary timescale of the post-AGB central star. Extending the spectral energy distribution (SED) to far-infrared wavelengths is an essential complement in constraining the disc characteristics. The long-wavelength flux is very sensitive to the amount of large grains, and thus the total dust mass, and grain-size distribution within the disc. The ultimate goal is to study the structure, formation and evolution of the very common discs around evolved binary stars, and constrain their impact on the evolution of the binary systems.

Lead Scientist: Clio Gielen

Allocated time: 14.9 hours

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Far-infrared spectroscopy of fullerene-containing

The very recent and unexpected detections of fullerenes (C60 and C70) and graphene (planar C24) in the H-rich circumstellar environments of transition sources evolving from the AGB to the PNe stage indicate that these complex molecules may be not so exotic and can form under conditions which are common to essentially all Solar-like stars at the end of their lives. This result has profound implications on our current understanding of the chemistry of large organic molecules because it demonstrates that formation of large fullerenes does not require a hydrogen-poor environment contrary to many theoretical and experimental expectations. Thus, fullerenes and fullerene-related species (e.g., multishell or endohedral fullerenes) might be ubiquitous in the Universe and continue to be plausible candidates to explain many astrophysical phenomena.

The simultaneous presence of fullerenes, graphene, and PAHs in H-rich circumstellar envelopes suggests that these carbon-based molecules may be formed as decomposition products of hydrogenated amorphous carbon grains (HACs). This HACs formation scenario seems to be supported by the strong correlation between the presence of fullerenes and the detection of the unidentified 30um feature, which could also be attributed to HACs. Interestingly, HACs display also a strong and broad feature around 60 um that can be detected by Herschel. We propose Herschel PACS spectroscopic observations of fullerene-containing sources evolving from the AGB to the PNe stage in order to test the HACs formation scenario. The Herschel PACS spectra will be examined for the possible presence of other fullerene-based molecules such as multishell fullerenes and will be compared with laboratory spectra of other fullerene-related molecules. The proposed observations will create a unique Herschel data set of high archival value and will be a step forward on our current understanding of the chemistry of large organic molecules as well as of the chemical processing of dust grains in space.

Lead Scientist: Domingo Anibal Garcia Hernandez

Allocated time: 15 hours

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Probing the mystery of water vapour in carbon-rich

Using the HIFI instrument, we propose to investigate further the puzzling - but widespread – appearance of water vapour in carbon-rich stars. Following up on our discovery that water vapour is present in the warm inner envelope of the carbon rich AGB star IRC+10216, we will carry out a sensitive search for the minor isotopologues, H2-17O and H2-18O. The abundances of these species will provide a critical test of competing models for the origin of the water vapour present in the inner envelope. If the production of water vapor is initiated by the photodissociation of CO by UV radiation, as proposed by Decin et al. (2010) and Agúndez et al. (2010), then enhancements in the H2-17O/H2-16O and H2-18O/H2-16O ratios are expected; however, if non-equilibrium chemistry initiated by pulsationally-driven shock waves is responsible - an alternative mechanism proposed recently by Cherchneff (2011) - then no such enhancement will be observed.

Lead Scientist: David Neufeld

Allocated time: 12.2 hours

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Measuring the Stellar Yields of massive stars with Herschel PACS

The pre-supernova mass loss from massive stars can significantly enrich the ISM in nitrogen. We propose to use Herschel to determine the masses of newly synthesised nitrogen in three Wolf-Rayet (WR) ejecta nebulae. We will obtain PACS line scans of 4 IR fine structure lines for each nebula, covering the [NIII] 57um and [OIII] 88um lines, along with the density sensitive [NII] 122 and 205um lines. From these we can calculate the total ionized mass of Nitrogen along with the 57um to 88um flux ratio which will yield the N2+/O2+ abundance ratio, equal to the N/O ratio for a wide range of nebular conditions, provided the exciting stars have effective temperatures above a certain threshold value. To satisfy this constraint, we have chosen WR ejecta nebulae whose exciting stars have spectral types of WN7 or earlier, or WC7 or earlier.

The total nitrogen mass can be derived from the total nitrogen flux over all the PACS IFU pixels for both N2+ and N+. This will allow the first comparisons to be drawn between derived nebular masses and stellar yield predictions for massive stars.

These measurements will also allow us to quantify the N abundance enhancements for a significant sample of WR nebulae. This has not been possible before, due to (a) the high extinctions to many WR stars, which lie very close to the Galactic Plane, hindering optical spectroscopy of their nebulae; (b) the fact that optically-based nebular N/O ratios rely on line ratios of the trace ions N+ and O+, necessitating large and uncertain ionization correction factors, whereas far-IR observations sample the dominant ions, N2+ and O2+, as well as N+.

Lead Scientist: David Stock

Allocated time: 5.1 hours

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When twins are not identical: a HIFI scan of IRAS15194-5115 the stellar twin of IRC+10216

Currently, our knowledge of the chemistry occurring in the stellar atmospheres and circumstellar envelopes of AGB stars is far from being complete. A most powerful method to study the circumstellar chemistry is broadband spectral scans of individual sources. We propose here to perform a spectral scan of the high mass-loss rate carbon star IRAS15194-5115 in selected HIFI bands. Lines will be identified and radiative transfer modelling will be performed for most of the detected species to provide circumstellar abundances and isotope ratios. The results will be compared with those of similar spectral scans of other types of evolved objects, in particular IRC+10216, to address the efficiency of various chemical processes and their dependence on the evolution of the object. A comparison between isotope ratios determined for IRAS15194-5115 and IRC+10216 will be of special interest since these stars have most likely taken different evolutionary paths along the AGB. The proposed observations will improve our understanding of AGB stars and their role in the chemical evolution of galaxies.

Lead Scientist: Elvire De Beck

Allocated time: 49.2 hours

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Warm gas around the C-stars with high expansion velocity. Unveiling the nature of these objects

The nature of the C-rich evolved stars with circumstellar envelopes (CSEs) with high expansion velocities (HVCs hereafter) is a mystery. The general theory of circumstellar dynamics, which explains velocity field for for CSEs of AGB, red supergiant (RSG) and yellow hypergiant (YHG) stars, fails to explain the properties of the CSEs of HVCs. Usually, very high expansion velocities only appear in very luminous stars, but the widely accepted Hot Bottom Burning processes (HBB) prevent RSGs to become C-rich. In fact, the standard theory predicts that C-rich RSG stars cannot exist. However, first results based on CO J=1-0 interferometric maps for the HVCs IRC+10401 and AFGL2233 show that the characteristics of these objects are very similar to that of the RSG/YHG stars. This supports the idea that HVCs are indeed C-rich massive evolved stars. Also, the chemistry found for these objects show a high abundance of C but also N, which agrees with this massive origin for HVCs.

We have also been granted with PdBI time to map the CO J=2-1 emission as well as low-J transitions of HCN, SiO, SiN and HCO+ among other molecules for the same sources. These observations will provide us important information about the kinematics and chemistry of the cold gas of the CSEs around these objects. However, massive evolved stars have CSEs significantly warmer those of their low-mass counterparts. Therefore studying the warm gas around HVCs is fundamental to obtain an accurate view of the nature, formation and chemistry of these objects.

We propose to use HIFI/Herschel to observe high-J transitions of 12CO and 13CO, as well as the emission of the water lines and ammonia for the HVCs AFGL2233 and IRC+10401. The 12CO and 13CO lines will help us to trace the warm gas of the CSE around these objects. The water observations will help to understand the formation of this molecule in (presumably massive) C-rich stars, while ammonia will provide important information about the abundance of nitrogen in these objects, and their nature.

Lead Scientist: Guillermo Quintana-Lacaci

Allocated time: 8.2 hours

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Additional Hpoint observations of large post-AGB sources from HIFIStars

One of the most spectacular phases in the evolution of intermediate mass stars takes place at the end of their lives. At the end of the AGB, the central star dashes across the HR diagram from the red giant to the blue dwarf region. At the same time, the spherically symmetric and slowly expanding circumstellar envelopes around AGB stars become planetary nebulae (PNe), displaying a large variety of shapes and structures far more complex. This transformation takes place at the very end of the AGB, and it is due to the interaction of fast and bipolar molecular winds with the fossil AGB circumstellar envelope. The origin of these post-AGB winds is still unclear, but we know that the resulting two-wind interactions are only active during a very short period of time, ~ 100 yr, but still they are able to strongly modify the kinematics of the nebulae and re-shape them. To better understand these processes we must study the warm molecular gas component of early post-AGB sources, both pre-planetary nebulae (pPNe) and young PNe. Herschel/HIFI is very well suited at this, because its spectral coverage, high velocity resolution, and superb sensitivity. For these reasons, 10 pPNe and young PNe were included in the KPGT HIFISTARS, were a large number of spectral lines are observed in a moderate number of frequency setups, but just at the central point. In many cases this is simply enough, since most post-AGB sources in HIFIStars are compact. However there are three cases in which the non spherically symmetric structures seen in the warm gas are larger than the telescope beam: OH231.8+4.2, NGC7027 and NGC6302. Therefore we propose to perform some additional points in these three sources in a selected sample of HIFISTARS frequency setups, were we have detected strong lines of CO, H2O, NH3 and OH. These observations are crucial to understand the kinematics and interactions traced by these warm gas probes, and gain insight in the intricate problem of the post-AGB dynamics.

Lead Scientist: Javier Alcolea

Allocated time: 18.1 hours

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Study of the cool forsterite dust around evolved stars

We propose to perform PACS range spectroscopy around the 69 micron forsterite dust feature of evolved stars. With the spectral resolution and sensitivity of PACS we are able to fit the profile of the forsterite feature which is very sensitive to temperature and composition of the grain. Based on a sample, observed in earlier GT programmes, of about 15 evolved stars, including OH/IR stars, post-AGB and PNs, we find that the olivine dust is purely Mg-rich and contains no iron. We find differences between the 'outflow' sources and sources which have disks, likely indicating a different formation history. The modelling of the mid-infrared bands (observed with SWS) in combination of the 69 micron band of the heavily obscured OH/IR stars allows to study the dust distribution and search for deviations from spherical symmetry or changes in mass loss history. With the new observations of a new set of 13 stars we want to substantiate our findings. The combination of SWS and PACS spectroscopy will provide a legacy dataset for future studies of crystalline dust.

Lead Scientist: Joris Blommaert

Allocated time: 3.1 hours

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Investigating AGB dust condensation conditions.

We propose to observe five AGB stars with low mass-loss rates using Herschel-PACS in spectroscopy mode between 51 and 73 micron. This wavelength range covers the high-excitation lines of CO and SiO that will allow us to determine the density and temperature of the inner wind where dust is being formed, and thus shed light on the actual physical conditions that determine the outcome of the dust formation process. The AGB stars with low mass-loss rates are particularly important in these studies since their IR dust spectra represent the first dust grains to nucleate, and moreover exhibit the largest diversity in dust condensation products. Our sample thus represents a subset of evolved stars that is indispensable to understand this condensation process, but that is at best highly underrepresented in currently approved evolved stars programs with Herschel. When combined with data from these existing programs, our observations will allow to make great progress in our understanding of the the dust condensation process in AGB stars. The small (2.4 hours) complementary program we propose here will thus greatly increase the scientific return of these programs.

Lead Scientist: Jan Cami

Allocated time: 2.4 hours

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A search for far-infrared HCN lasers in C-rich evolved stars

The pumping mechanisms for astrophysical masers are often complex, involving infrared line opacities, collisions, and infrared line overlaps. Hence, the interpretation of the observed emission requires detailed modelling. In spite of these potential problems the masering lines can, however, give useful information on the physical conditions of the regions where the masers are produced, i.e., the innermost zones of circumstellar envelopes.

While SiO, OH, and H2O masers are a common feature of O-rich stars and were detected more than forty years ago, strong maser emission in C-rich stars has been detected only in some vibrational lines of HCN, and in some high-J lines of the ground state of SiS.

In the line survey of IRC+10216 with HIFI we have discovered several lasers involving the pair of vibrational levels 4v2/v2+v1. The strongest ones are above 1 THz and carry information of the 1-3 stellar radii zone. Several masers have been also found in some lines of the v2 bending mode between 500 and 900 GHz.

We would like to carry a systematic search for HCN masers in C-rich evolved stars by observing selected frequency ranges covering from the v1 to the 3v2 vibrational states (typically a range of 15-20 GHz) for each rotational line within the SIS receivers of HIFI (from J=6-5 up to J=13-12). The pummping mechanism can be different in each object depending on the dust opacity. High mass loss rate stars will have their 3 micron emission absorbed near the star by the large amount of dust produced in these objects. The pumping of HCN will be done mainly through the bending mode at 14 microns. However, in low mass loss rate the 3 micron photons will populate the v1, v3 and combination bands allowing a much complex pumping pattern.

This study will permit to characterize HCN laser emission in C-rich stars and their pumping mechanisms. It will also complement our recently granted high angular resolution observations of HCN with ALMA in cycle 0.

Lead Scientist: Jose Cernicharo

Allocated time: 20.3 hours

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TIME VARIABILITY OF THERMAL MOLECULAR LINE EMISSION IN IRC+10216

We have found during our GT line survey of IRC+10216 and the search for hydrides (OT1 proposal) that some molecular lines present a strong intensity variation with time due to the role of infrared pumping. For some lines the intensity change in six months reaches a factor 3 (CCH). We have checked that the effect is not instrumental and than it arises from physical processes ignored so far in the radiative transfer models.

We propose to observe the CCH and HNC lines within bands 1a-5b of HIFI every four months (three observing slots) to allow a detailed study of the variation of thermal molecular emission, and dust emission, in this prototype of AGB C-rich object. The settings will also provide, as a bonus, many lines of SiO, SiS, CS, HCN, CO and 13CO for which intensity variations of up to 30% have been found. In addition, a few specificc settings for HCN and CO will complete the observations. SPIRE and PACS observations will complement, with lower spectral resolution, the whole spectrum of each of these molecules and will provide a global view of the total intensity change of these lines with time. A crude estimate of the distance could be also obtained from the observed time lags between the blue and red parts of the line profiles observed with HIFI.

Lead Scientist: Jose Cernicharo

Allocated time: 16.5 hours

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Understanding Shock Oxygen Chemistry in Interacting Supernova Remnants

Supernova remnants interacting with dense moelcular clouds provide astrochemical laboratories to study heating and cooling of the dense ISM, shock chemistry, destruction and sputtering of dust, and the reformation of molecules. Water is expected to be a major coolant for shocks into dense gas, yet the number of remnants in which IR lines of hydroxyl and water are detected is very limited. We propose Herschel PACS, SPIRE and HIFI observations of three remnants with particularly high shocked gas densities, high dust and IR line luinosities, and extreme ionization environments. The scientific objectives of this proposal are: (1) to determine the abundance and excitation of oxygen-bearing molecules, and (2) to study the effects of variable ionization sources on oxygen chemistry in dense molecular gas shocked by powerful supernova remnant blast waves.

Lead Scientist: John Hewitt

Allocated time: 10.1 hours

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The Origin and Nature of the Emission Nebulae around Symbiotic Stars

There is much controversy concerning the ionized nebula that produces the radio through FIR emission from symbiotic stars. The goal of the proposed Herschel observations is to test two popular models for this emission; whether it is produced by a wind from the red giant that is photoionized by Lyman continuum photons from the hot WD (STB) or it comes from plasma that is shock heated as the winds from the two stars collide by constraining the submm SED and measuring the free-free turnover frequency of the ionised component. These two models predict distinctly different shapes for the submm portion of the SED and different dependence of the turnover frequency on binary separation. Thus, submm photometry of a diverse sample of symbiotic stars with know binary parameters that only Herschel can perform is an ideal way to quantitatively test and discriminate between these models (as well as motivate new ones). In terms of astrophysical significance, determining the origin of the radio-through-FIR emission from symbiotic stars has implications for the nature and geometry of mass transfer in wide binaries, mass loss from accreting compact objects, the shaping of asymmetric nebulae around binary stars (including binary planetary nebulae), and the likelihood that symbiotic stars can explode as type Ia supernovae.

Lead Scientist: Jennifer Sokoloski

Allocated time: 17.1 hours

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Herschel observations of extreme OH/IR stars

A subset of Asymptotic Giant Branch (AGB) stars loses mass at a very high rate. The dust effectively shields the radiation from the central star, allowing water-ice to condense onto existing silicate grains. Through the mass loss process, these stars provide a siginificant fraction to the gas and dust mass return to the interstellar medium. They are thought to be massive intermediate-stars (>5 Msun), close to the end of their AGB evolution.

Using Herschel to obtain the full spectral coverage from 50 to 670 micron with PACS and SPIRE, we will obtain spectra of gas-phase H2O lines and derive the H2O abundance by modelling these lines, thought to be the main cooling agent in O-rich AGB star. This, combine with a few selected high resolution observations of CO lines with HIFI will allow us to explore both the temperature and density structures of the circumstellar envelope (CSE). These observations will enable us to study the H2O abundance as a function of distance from the star as well as explore the superwind region close to the central star and detect any departure from spherical symmetry in the structure of the CSE. We will also get a handle on the stellar mass from 12C/13C and the wealth of the molecular emission lines in this wavelength range will be used to study the ongoing chemistry in highly obscured circumstellar envelopes of AGB stars.

Lead Scientist: Kay Justtanont

Allocated time: 30.8 hours

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Unraveling the mechanism for the creation of warm water vapour in the sooty outflow of luminous carbon stars

One of the highlights of the first year of Herschel's science program was the discovery of warm water vapour in the envelope around the carbon-rich evolved star IRC+10216 (Decin et al. 2010, Nature). The water abundance derived for this carbon-rich AGB star is 4 orders of magnitude larger than the photospheric abundance expected under thermochemical equilibrium. This huge discrepancy had led to the suggestions of several possible origins for the water vapor. The relative strengths of the high-excitation water lines in the Herschel data indicate the presence of warm water vapor close to the star. Only two, still competing, theories are consistent with the existence of warm water vapor.

Very strikingly, water vapour was later on detected in eight other carbon-rich evolved stars, i.e. every star in a small sample surveyed with HIFI. Much to our surprise, recent PACS observations did not reveal the presence of water in one other carbon-rich target (AFGL3068). So far, this is the only carbon-rich star without water detection. Currently, it is absolutely unclear which are the key physical and chemical parameters determining if water will be formed or not.

The currently available Herschel observations show a very strong bias toward high mass-loss rate targets. Additionally, Mira-type pulsators with high pulsational amplitudes are favored over Semi-Regulars. We aim to extend the small HIFI sample with 10 new targets, carefully selected to cover all (circum)stellar properties thought to be relevant for the creation and excitation of water. We will observe 5 ortho-water lines and 1 para-water line, covering different excitation energies. That way, Herschel will provide us with high-quality data being key to unravel the physical and chemical conditions prerequisite for the formation of water in the sooty outflow of luminous carbon stars.

 

 

Lead Scientist: Leen Decin

Allocated time: 21.8 hours

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Revealing the nature of the remarkable object IRAS 19312+1950

The IRAS source 19312+1950 is a peculiar bipolar nebula that has eluded firm characterisation since its discovery. It exhibits maser and outflow properties similar to a massive O-rich AGB star, but shows molecular species such as CH3OH and HC3N that are more typically found in molecular clouds or YSOs. The source is surrounded by remarkable NIR nebulosity and has an unusual SED. The puzzle over the true nature of this object is confounded by our Spitzer IR spectrum that shows amorphous silicates and CO2 ice, but also emission from what may be crystalline silicates. In order to understand the physical properties of the gas and dust in the vicinity of IRAS 19312+1950, we propose to perform Herschel observations of emission from CO and H2O across a broad wavelength range from sub-mm to IR. Observations of transitions from a variety of energy levels will allow us to probe different temperature and density regimes within the source, from which we will construct a picture of its physical structure through radiative transfer/excitation modelling. HIFI observations will allow us to separate the broad and narrow molecular line components and PACS IFU mapping will provide crucial spatial information on the physical structure of the source. We also propose to perform a PACS SED scan to better characterise the source SED, and measure diagnostic spectral features of the dust and gas such as the forsterite 69 micron band, the OI 63 micron line and the N II 122 and 205 micron lines. The combination of these unique observations will help solve the puzzle of the nature of this peculiar object.

Lead Scientist: Martin Cordiner

Allocated time: 9.2 hours

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Mass Loss from Classical Cepheids and their Progenitors

We propose Herschel PACS and SPIRE imaging of a sample of 26 Classical Cepheids, to map their far-IR circumstellar emission. All targets have been previously imaged with Spitzer/IRAC and MIPS. Herschel data will allow to characterize the nature of the nebulosity for all sources, and differentiate it from "Galactic Cirrus'" background emission and local ISM clouds. This will provide an accurate and unbiased estimate of Cepheid mass loss rates, crucial for resolving the "Cepheids mass discrepancy'".

Lead Scientist: Massimo Marengo

Allocated time: 26 hours

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The origin of far-infrared emission in the supernova 1987A

The detection of 0.4-0.7 solar masses of dust in the ejecta of SN 1987A is one of the most important results of the Herschel survey of the Magellanic Clouds (HERITAGE). It has provided astronomers with strong observational evidence that supernovae (SNe) can be major contributors to the dust in the local interstellar medium (ISM) and to the massive amounts of dust present in ultraluminous infrared (IR) galaxies in the early universe. The discovery has drawn some scepticism regarding the validity of the interpretation, i.e., that the 100 to 350 micron fluxes can indeed be attributed to continuum emission from dust. We propose PACS and SPIRE deep spectral observations to definitively determine the nature of the far-IR continuum emission and the contribution of molecular and atomic line emission to the observed broad band fluxes that we used to derive the dust mass. In addition to validating the dust model, any observed line emission will be used to determine the molecular and atomic abundances and the physical conditions in the material ejected from the SN.

Lead Scientist: Mikako Matsuura

Allocated time: 22.3 hours

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Atomic and Molecular Gas Observations of Supernova Remnants in the Magellanic Clouds

We propose PACs and SPIRE spectroscopy of three core-collapse supernova remnants (SNRs) in the Large and Small Magellanic Clouds (LMC, SMC): 1E0102-7219, N132D and N49. They are chosen to have a range of ages and degrees of interaction with nearby molecular clouds. We will use the spectroscopy to 1) constrain shock models, 2) judge the line contamination of broadband fluxes used to measure dust mass, 3) determine carbon and oxygen abundances and gas masses and 4) understand the CO ladder in cases where SNe shocks interact with molecular clouds. SNRs play a fundamental role in the evolution of galaxies: their ejecta drives the chemical evolution of the interstellar medium (ISM), and the energy liberated in their explosion drives the shock waves that generate bulk motions in the ISM, accelerate cosmic rays, regulate the star formation rate, and alters the size and properties of interstellar dust. In order to understand the life cycle of dust, which is the overarching science goal of the HERITAGE key program on the LMC and SMC, we must investigate SNR shocks in both the supernova ejecta and the ISM. SNRs radiate from radio to X-ray wavelengths, but far-infrared (FIR)/submm observations are crucial both because shock heated dust is visible in these bands and because the FIR lines in many cases dominate the cooling in SNRs. For the first time, Herschel provides the necessary sensitivity and spatial resolution to map LMC and SMC SNRs in several critical cooling lines in SNR shocks: [O I] 63um, [C II] 158 um and [O III] 88 um with PACS spectroscopy, and CO rotational lines with SPIRE/FTS. The atomic fine-structure transition lines in the FIR are important shock diagnostics particularly for the lower densities (~50-500 cm^{-3}). The submm molecular lines will provide critical information on the interaction of SNRs with neighboring molecular clouds. Comparison of our results with Herschel studies of Galactic SNRs will reveal potential dependencies of SNR evolution on metallicity of the ISM.

Lead Scientist: Margaret Meixner

Allocated time: 10.9 hours

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Clumping in OB-star winds

Massive stars, their nature and evolution, play a important role at all stages of the Universe. Through their radiatively driven winds they influence on the dynamics and energetics of the interstellar medium. The winds of OB stars are the most studied case. Commonly, the mass-loss rates of luminous OB stars are inferred from several types of measurements, the strengths of UV P Cygni lines, H-alpha emission and radio and FIR continuum emission. Recent evidence indicates that currently accepted mass-loss rates may need to be revised downwards when small-scale density inhomogeneities (clumping) are taken into account. We argue that only a consistent treatment of ALL possible diagnostics, scanning different parts of the winds, and analyzed by means of ‘state of the art’ model atmospheres, will permit the determination of true mass-loss rates. To this end we have assembled a variety of multi-wavelength data, but one crucial observational set is missing: far-IR diagnostics of free-free emission, which uniquely constrain the clumping properties of the wind at intermediate heights. We propose, therefore, to use PACS photometric mode to fill this crucial gap, studying the 70 and 110 micron fluxes of a carefully selected sample of 29 O4-B8 stars. These observations will provide the missing information to derive the clumping properties of the entire outflow, to understand the wind physics, and to obtain reliable mass−loss rates.

Lead Scientist: Maria del Mar Rubio

Allocated time: 12.8 hours

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Old and slow Galactic bullets: Tracing the dust in turbulent interaction regions due to AGB stars traversing interstellar space

Herschel has revealed that wind-ISM interaction regions and detached rings occur frequently around evolved stars. These interaction regions are observed to yield various shapes and sizes which can be divided in four main morphological classes. To first order the distance between the bow shock apex and the star is set by the stellar properties (mass-loss rate, wind velocity, peculiar motion) and the properties of the local ISM (density and temperature).

The primary objective of this proposal is to characterize and understand the interaction of stellar mass-loss with its immediate surroundings by studying the direct emission cold dust as it is trapped in the interaction region between the stellar wind and the ISM. Thus we can follow the fate of circumstellar dust, use the observed morphologies as tracers of the ISM and of the later stages of stellar evolution. Also we aim to elucidate effects of magnetic fields, binarity, and circumstellar chemistry on the shaping of these interaction 'bow shock' regions.

Herschel is uniquely equiped to trace the cold dust that is trapped in the studied interaction zones. These goals can be achieved best by performing a sufficiently comprehensive PACS imaging survey of nearby AGB stars.

Lead Scientist: Nick Cox

Allocated time: 36 hours

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Capturing missing evolved stars in the Galactic plane

We discovered more than 400 compact shells in the MIPSGAL 24 microns survey of the Galactic plane. About 15% of all these objects were already known as planetary nebulae, supernova remnants, Wolf-Rayet stars, and luminous blue variables. The unknown bubbles are expected to be envelopes of evolved stars that could account for the ``missing’’ massive stars in the Galaxy. Indeed, recent spectroscopic follow-ups in the near-IR and mid-IR have revealed several dust-free planetary nebulae with very hot central white dwarf and significantly increased the number of WR and LBV candidates.

Our OT1 Priority 1 proposal just provided us with a first observation in the PACS-SED B2A mode of one object, revealing only a strong [N II] 122 microns line. Without further spectral information, identification and modeling of the target are impossible. However, analysis of the PACS and SPIRE data from the HiGal survey has recently enabled us to measure much higher detection rates of the shells in the far-IR than with MIPS 70 microns. We are thus very confident that dust features and/or gas lines can be detected with the PACS and SPIRE spectrometers. Therefore, we request complementary PACS-SED B2B and SPIRE-FTS observations on our OT1 sample.

The complete far-IR/submm spectrum of each target will allow its unequivocal identification thanks to comparison with spectra of known evolved stars from the MESS key program. We will also model with much detail the different phases of the envelopes, thanks to our expertise in circumstellar envelopes, dust models and photoionization codes.

Lead Scientist: Nicolas Flagey

Allocated time: 9.8 hours

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A Search for Missing Dust in Nearby Core-Collapse Supernovae

We propose to carry out a Herschel/PACS far-infrared (70 and 100 um) survey for thermal emission from cold (T < 150 K) dust in eleven observable core-collapse supernovae (CCSNe) known to have occurred within 4 Mpc over the past 20 years. The source of the large amounts of dust observed in high redshift galaxies has remained uncertain for nearly 40 years. Despite the success of theoretical models in condensing dust within supernova ejecta, only a handful of supernovae show direct observational evidence for dust condensation, and these examples all yield 2-3 orders of magnitude less dust than predicted by the models. The recent discovery of a large (1 M_sun) reservoir of newly formed, cold (20 K) dust in SN 1987A has revolutionized our outlook by serving as a reminder that a significant portion of newly formed dust likely exists at colder temperatures. Aside from SN 1987A, however, cold dust has only been observed in nearby supernova remnants (SNRs) due to the limited sensitivity and resolution of longer wavelength instruments, such as Spitzer/MIPS. We show in this proposal that, assuming a dust mass similar to that observed in SN 1987A, Herschel/PACS will be sensitive to cold dust down to 40 K and will provide meaningful upper limits in cases of non-detections. Given this is Herschel's last round of proposals, this is the only and best opportunity to perform the first search for cold dust in a sample of extragalactic SNe. In only 14.8 hours, we can obtain two color photometry to constrain the dust temperatures and masses in the eleven CCSNe.

Lead Scientist: Ori Fox

Allocated time: 14.8 hours

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Oxygen abundances in carbon-type Wolf-Rayet stars from PACS scan spectroscopy

This is a resubmitted priority 2 OT1 programme to scan [OIII] 88micron with PACS for a sample of Milky Way carbon and oxygen sequence Wolf-Rayet stars. The proposed observations, requiring 7.7hr will: (i) enable reliable oxygen abundances to be determined for WC stars for the first time, testing evolutionary predictions; (ii) refine the degree of clumping in the outer stellar winds of these stars derived from existing ISO/SWS or Spitzer/IRS datasets. The requested line spectra are unique to PACS and cannot be acquired with another instrument for these targets.

Lead Scientist: Paul Crowther

Allocated time: 7.7 hours

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Herschel imaging of SNR G292.0+1.8: Cas A's older cousin?

It is well-known that supernovae play a key role in creating and distributing elements throughout the universe, but less well-known is their contribution to the overall budget of dust in the ISM. Theoretical models suggest that core-collapse supernovae should produce large quantities of dust, but observational evidence for this is still debated. Even if significant quantities of dust are produced, does it survive the passage of the reverse shock to enter the ISM? Does the forward shock destroy all dust that it encounters? Near and mid-IR observations with Spitzer and AKARI have begun to answer these questions, but the long wavelength cameras of Herschel are necessary for a complete picture. We propose detailed observations of G292.0+1.8, a large Galactic supernova remnant (SNR) that has been called the "older cousin" of Cassiopeia A. One of the few known oxygen-rich remnants, G292 is a 3000 year old textbook example of a core-collapse SNR expanding into its own circumstellar medium (CSM), the wind of a red giant. It is one of the most well-studied SNRs at all wavelengths, from radio to X-rays. At 8' in diameter, it is large enough for the emission from the forward-shock CSM to be well-separated from that of the reverse-shocked ejecta, yet it is still small enough to be fully covered by Herschel in a reasonably small amount of time. We will obtain PACS and SPIRE imaging of the entire remnant. PACS observations will be sensitive to forward-shocked material, while SPIRE data will tell us whether large amounts of ejecta dust are present in the remnant. We will use the far-IR data in conjunction with X-ray and optical data to obtain a complete picture of the dynamical evolution of the remnant, and advance our understanding of the nature of dust in the universe.

 

Lead Scientist: Parviz Ghavamian

Allocated time: 7 hours

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The stellar wind of two Galactic Cepheids: a key to solve the evolutionary/pulsational mass discrepancy

Thanks to their Period-Luminosity law, Cepheids are among the most important class of stars. The recent discovery of circumstellar envelopes (CSEs) around Cepheids is an indication that many Cepheids, if not all, are surrounded by CSEs. The bright classical Cepheids RS Pup and delta Cephei are particularly interesting members of their famous class of variable stars, as they are known to be surrounded by large CSEs. Following our PACS and SPIRE imaging observations of the dusty circumstellar envelopes of these two Galactic Cepheids in OT1, we request spectroscopic observations of the same two targets to measure the physical conditions and composition of their stellar winds. Our scientific goal is to determine the physical conditions in the stellar winds, and pinpoint, in combination with other observations, the mass loss rate of these two benchmark stars. There is still today a significant discrepancy between the Cepheid masses estimated from evolutionary modeling, and pulsation modeling. The evolutionary modeling systematically predicts higher masses than the ``instantaneous", dynamical pulsating masses. This may indicate that massive stars experience significant mass loss episodes during their Cepheid phase. We aim at testing the hypothesis that pulsation driven stellar winds occur during the crossing of the instability strip, and are at the origin of this discrepancy.

Lead Scientist: Pierre Kervella

Allocated time: 7.3 hours

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Uncovering Supernova Ejecta: Herschel Spectral Mapping of G21.5-0.9

G21.5-0.9 is a supernova remnants with bright pulsar wind nebulae powered by a young, active pulsar, and for which IR observations provide evidence of the interaction between the pulsar wind nebulae and the inner ejecta of the remnants. Combined with known properties of the central pulsar, and broadband images and the spectral energy distribution, information derived from spectroscopy of the interaction regions can place strong constraints on the evolution of this systems and on its progenitor star. Here we propose Herschel spectral mapping studies of G21.5-0.9 in order to obtain velocity and composition measurements of the inner ejecta in order to investigate the progenitor properties and the dynamical evolution of the system.

Lead Scientist: Patrick Slane

Allocated time: 4.7 hours

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Molecule formation in planetary nebulae

We propose to observe a sample of highly evolved planetary nebulae that we believe to have ongoing molecular chemistry inside dense knots that formed only a few thousand years ago inside recombining gas. The proposed Herschel observations will allow us to either prove or disprove this new, still controversial, formation scenario of the knots. If proven correct, these knots will allow us a unique opportunity to test the theory of time-dependent molecular chemistry. Two famous examples of such objects are the Helix nebula (NGC 7293) and the Ring nebula (NGC 6720). They have knots that are currently embedded in the ionized gas as the ionization front has moved outwards since the knots formed. The Helix nebula has very strong H2 emission coming from the knots. A static photoionization model cannot explain this emission, but a hydrodynamic model can. This model indicates that the knots are quickly eroded by the radiation field of the central star. This poses a problem for rival theories as they assume that the knots formed much earlier, and must have survived through the entire photoionized phase of the nebula. We believe that the knots cannot survive that long in such a harsh environment and formed after the central star entered the cooling track and the nebulae started to recombine. In order to prove this we need more accurate models of the advection flows off the knots that need to be constrained by Herschel observations of the full CO emission line spectrum. To sample various stages of evolution, we have searched for evolved planetary nebulae with knots which were sufficiently bright. After removal of duplications with earlier proposals, we were left with a sample of 5 planetary nebulae, including the Helix nebula. We propose to obtain SPIRE full range spectroscopy and PACS deep line scans on individual CO lines, allowing us to observe the CO spectrum from the 4--3 line up to the 24--23 line.

Lead Scientist: Peter van Hoof

Allocated time: 15.9 hours

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Water Legacy of HIFI -- Observing the richest water spectrum in evolved stars: W Hya

In order to investigate the relative impact of water production in evolved stars on the ISM water budget and in order to understand accurately the water excitation in circumstellar environments, we suggest to extend the observations taken for the HIFISTARS Guaranteed Time Key Program (GTKP, P.I.~V.~Bujarrabal) for the AGB star W Hya and to complement the PACS/SPIRE observations taken within the framework of the MESS GTKP (P.I.~M.~Groenewegen). This will provide the most complete inventory of water vapour emission lines yet observed, providing Herschel with its very own water legacy. In addition to the observations already taken, the proposed number of 17 transitions for a total of 18.7 hours of observation time allows us to (1) significantly improving our knowledge on water excitation and (2) study the thermophysical structure (water cooling, wind acceleration and the dust-gas reciprocal influence) in the CSE of W Hya in all detail. In turn, we can pinpoint the radial water abundance profile, which provides information on the chemical processes responsible for forming water in an AGB environment, leading to a template for other unobserved or partially observed oxygen-rich AGB stars. W Hya is a low mass-loss rate oxygen-rich semi-regularly pulsating AGB star at a distance of 78 pc. It has a relatively well understood geometry, a large water abundance, and more importantly it is the evolved star with the strongest thermal water emission detected to date, as shown by the observations carried out for the HIFISTARS GTKP.

Lead Scientist: Robin Lombaert

Allocated time: 18.7 hours

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Probing the molecular disk in Y Gem: an AGB star with variable UV emission signifying accretion onto a binary companion

We propose to observe CO J=6-5 and 9-8 line emission from a cool AGB star, Y Gem, which, in dramatic contrast to most objects in its class, has relatively strong and variable FUV and NUV fluxes - evidence of variable accretion of matter onto an accretion disk in a binary system. We found Y Gem as a UV source serendipitously, while combing the GALEX archive as part of a project to look for hot binary companions to cool AGB stars. This object may represent the earliest phases of an AGB star with a growing accretion disk which will produce collimated jets that are widely believed to sculpt the round circumstellar envelopes of AGB stars into bipolar planetary nebulae. It may evolve into a member of the class of post-AGB objects which show no extended outflows, but only circumbinary disks. HIFI observations of high-J CO lines are needed to probe the warmest and innermost circumstellar regions where the hypothesized accretion disk resides and jet launching may occur. Furthermore, the proposed CO observations, together with our existing CO J=2-1 data, will allow us to accurately constrain the CO excitation temperature, and the optical depths of the CO lines and thus the total mass of the emitting region. The disk or torus mass will provide an important constraint on its formation process (e.g., common envelope evolution or Bondi-Hoyle wind-accretion/ Roche lobe overflow.)

Lead Scientist: Raghvendra Sahai

Allocated time: 3.1 hours

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Carbon abundances in Galactic bulge planetary nebulae

Carbon (and nitrogen) abundances, which can be used to constrain stellar evolution theory, are not well known in Galactic bulge planetary nebulae because the lines used are in the ultraviolet and are weak and difficult to measure accurately. We propose to measure the C(+) abundance in a selection of low ionization PNe where it is the dominant ion, by measuring the CII line at 157 microns. The nitrogen line at 122 microns is used in conjunction with the line at 6584 angstroms to accurately determine the electron temperature, which not only will make the nitrogen abundance more accurate, but will enable abundances to be determined for other ions measured in the visual spectrum.

Lead Scientist: S.R. Pottasch

Allocated time: 4.9 hours

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How Molecules and Dust form/survive in C- and O-depleted Circumstellar Ejecta

Eta Carinae is a lynchpin between mass ejection of highly evolved massive stars and the enriched interstellar medium. Eta Carinae's ejecta, created in the 1840s and 1890s, is known to be nitrogen-rich with carbon and oxygen 50-100 times depleted compared to solar abundances. Most of the C,O depletion is caused by CNO processing broken by conduction from the stellar core to the outer envelope for stars greater than 60 solar masses. More depletion occurs during molecular/dust formation in the high temperature/densities of the mass ejection. From HST/STIS spectra, we have determined many iron-peak metals are greatly overabundant. It is unclear what molecules and dust form during mass ejections, and can survive in the ejecta, especially since the C, O depletions limit formation of oxides and carbides that normally precipitate onto cores of dust. Silicates and alumina grains are known to be present in the environment. This program may infer whether metallic grains are also present. 

We propose to inventory atoms and molecules in the ejected material. The Doppler velocities of this material range over +/-500 km/s in several distinct components. Although our OT1 program has not been fully observed yet, the PACS spectra in hand give hints of what can be detected with the higher HIFI velocity resolution. Emphasis will be placed on trying to understand the unusual chemistry and dust precursors arising in an exceptionally nitrogen-rich and carbon/oxygen-deficient environment. We request further HIFI scans to probe the numerous unidentified and anticipated spectral features, in particular N-bearing molecules, and to address the total mass of the ejecta.

Lead Scientist: Theodore Gull

Allocated time: 16.1 hours

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Unraveling the Outburst and Quiescent Emission Properties of the Enigmatic Binary Cir X-1

Cir X-1 is an unusual X-ray binary in that it has large scale outbursts every 16.58 d. The cause of these outbursts is believed to be due to episodic mass transfer events that occur when a non-degenerate companion, in a highly eccentric orbit around a compact object, passes through periastron. In the 1970's, Cir X-1 was in a very luminous "high state" that resembled the outbursts of X-ray transients with black hole primaries. At this time, the maxima of the outbursts reached to K = 7.7, even though Cir X-1 is more than 5 kpc from the Sun! In the early 1980's, Cir X-1 was observed to show "Type I" X-ray bursts that indicated the compact object was a neutron star. Unfortunately, these events stopped shortly after they began, only resuming recently. In the 1990's, however, the X-ray behavior of Cir X-1 was finally confirmed to be due to accretion onto a neutron star primary. What makes Cir X-1 especially interesting is the presence of an ultra-relativistic jet. This jet is seen in both the X-ray and radio, and extends to parsec scales. There is also evidence that this jet is precessing. If true, then Cir X-1 is the neutron star counterpart to the unusual high mass X-ray binary system SS433 (where the compact primary is a black hole). We propose to obtain PACS + SPIRE photometry of Cir X-1 both in outburst, and during quiescence, to identify the dominant emission processes that occur during these phases. These observations will allow us to determine if the outbursts are due to a synchrotron jet that simply turns on during outburst or, alternatively, whether the jet is always present, but precesses across our line-of-sight once every 16 d. Precessing jets are seen in pre-main sequence stars, other X-ray binaries, and AGN. Given its extensive observational history, Cir X-1 is the ideal source for understanding the processes responsible for launching relativistic jets. Our program requests 2.5 hr of Herschel time.

Lead Scientist: Thomas Harrison

Allocated time: 2.5 hours

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Disks as stellar-merger remnatns: the case of V4332 Sgr and V1309 Sco

Red novae are a new class of eruptive variables observed in the Milky Way and Local Group galaxies. Their eruptions are thought to be due to stellar collisions and subsequent mergers. Only recently it has been convincingly shown that one of the red novae, V1309 Sco, was a contact binary prior to its eruption. Thanks to accurate photometry from OGLE going years before the eruption we could trace the shortening of the orbital period in V1309 Sco as it was evolving toward a merger. The OGLE light curve suggests that a disk or disk-like feature was formed just before the main outburst of V1309 Sco. A disk is known to be present around another, older red nova V4332 Sgr. Such disks are expected to form during and after a merger as they accumulate angular momentum that has to be lost during the spiralling-in process. It has been suggested that from such disks 'second generation' of planets can form. This hypothesis was introduced to explain the nature of the somewhat peculiar planets, called inflated hot Jupiters. We propose to perform PACS and SPIRE photometry of the two red novae, V1309 Sco and V4332 Sgr. The far-infrared fluxes will be used to construct full spectral energy distributions of these objects, from optical to radio wavelengths. Detailed radiative-transfer modelling of the disks will determine their main parameters, i.e. mass, sizes, and dust properties. The disk parameters will be used to study the physics of a merger event. Of main interest are the disk mass and the angular momentum deposited in the star and the disk. Knowing the disk mass will allow us to test the planet formation hypothesis. By comparing the old disk of V4332 Sgr with the newly formed one in V1309 Sco, and by analysing multi-epoch variability in the disk of V4332 Sgr (the Herschel data will be compared to older Spitzer and AKARI data) we hope to gain insight on the disk formation process.

Lead Scientist: Tomasz Kaminski

Allocated time: 1.1 hours

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Far-Infrared Observations of the Remarkable Mid-Infrared Source IRAS15099-5856

We propose to make Herschel observations of the infrared source IRAS 15099-5856 associated with a supernova remnant (SNR) detected by AKARI and Spitzer observations. IRAS 15099-5856 is the first infrared source that shows crystalline silicate features associated with a SNR or its progenitor. It offers a unique opportunity to study the mass loss and the succeeding explosion in a massive progenitor system, for which we have only little knowledge at present. Far-infrared (FIR) imaging and spectroscopic observations with PACS and SPIRE will reveal the true nature of IRAS 15099-5856 and its extended structures for the first time in the FIR and will certainly deepen our understanding of the mass loss of a massive star prior to a SN explosion.

Lead Scientist: Takashi Onaka

Allocated time: 8.7 hours

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PACS Observations of Supernova Ejecta and Dust in the Composite Supernova Remnant Kes 75

Composite supernova remnants present us with a unique opportunity for detecting and characterizing supernova ejecta and dust as it is being illuminated by the central pulsar wind nebula. Kes 75 is a Galactic remnant consisting of a pulsar wind nebula and a partial thermal shell. It has been observed at radio, infrared, and X-ray wavelengths, but the nature of the observed emission and the progenitor star remains unknown. High absorption towards the remnant has prohibited any optical searches for supernova ejecta and has made the results from X-ray observations inconclusive. However, the 70 micron image from the Herschel Hi-GAL key project revealed emission around the pulsar wind nebula that most likely originates from the interaction of the nebula with supernova ejecta and dust. Observations with Herschel are currently the only means of detecting this ejecta and constraining the total mass of swept up gas and dust in Kes 75. We therefore propose Herschel PACS imaging and spectroscopy of the pulsar wind nebula and shell in Kes 75 in order to determine the remnant's properties and understand the nature of the supernova progenitor.

Lead Scientist: Tea Temim

Allocated time: 10.3 hours

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CI and CII in the circumstellar environment of the carbon-rich AGB star Y CVn

We propose to map the extended circumstellar environment of the carbon star Y CVn in the sub-mm lines of CI and CII. The high spectral resolution of HIFI will allow us to analyze in detail the line profiles and to separate the 12C from the 13C emissions. The comparison with the data obtained with the VLA in the HI line at 21 cm will allow us to disentangle the thermal broadening from the kinematic broadening. We will thus get access to the temperature profile in the circumstellar envelope and be able to constrain the cooling of circumstellar matter. The data will also provide the 12C/13C ratio and the carbon ionization state as a function of distance to the central star.

Lead Scientist: Yannick Libert

Allocated time: 21 hours

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