Herschel links Star Formation to Sonic Booms

Interstellar cloud IC5146
Interstellar cloud IC5156 containing the Cocoon Nebula and a dense network of filaments. Click for more information.

Astronomers using ESA’s Herschel Space Observatory have found that filaments of gas and dust in interstellar clouds have a preferred size, implying that they have formed as a result of interstellar sonic booms travelling through the Galaxy.

The filaments are huge, stretching for tens of light years, and Herschel has shown that newly-born stars are found in the densest parts of them. One such filament in the constellation of Aquila contains around 100 infant stars. The very cold gas and dust in interstellar space can only be observed with far-infrared light. While such filaments have been seen before, the high resolution of Herschel has allowed astronomers to measure their widths for the first time.

It is unusual in astronomy for objects to have a fixed size – planets stars and galaxies all come in a wide range of sizes – so it was expected that the filaments would be seen to have a range of widths. The team analysed 90 filaments in three regions of the sky. “Curiously, our study shows that all interstellar filaments detected in the three regions tend to have a typical width of about 0.3 light years”, commented Doris Arzoumanian, from CEA, the lead author on the paper describing this work. For comparison, this is around 20,000 times the distance between the Earth and the Sun, or around 1/12th of the distance to the nearest star. "These findings highlight that something must be going on at this particular scale", she adds.

By comparing with computer simulations, the astronomers have concluded that the filaments may be formed when slow shockwaves dissipate in the interstellar clouds. The shockwaves are the result of the energy produced by exploding stars, which cause a great deal of turbulence in the surrounding regions. These shockwaves travel through the Galaxy, sweeping up gas and dust and forming the dense filaments we see today.

Polaris Flare
The Polaris Flare in Ursa Minor, showing more filaments. While no stars are forming here, the filaments are around the same width as those un IC5146. Click for more information.

Since the interstellar clouds are extremely cold, at around 10 degrees above absolute zero (or -263 Celsius), the speed of sound is relatively slow – at just 200 m/s (450 mph), compared with 340 m/s (760 mph) at sea level here on Earth. This means that the slow shockwaves are the interstellar equivalent of sonic booms. As they lose energy in the clouds, they leave behind these tenuous filaments of gas and dust.

Other models have previously linked the formation of the filaments to gravitational collapse and the effect of magnetic fields, but without these observations the distinction was not possible to make. Prof Derek Ward-Thompson, Cardiff University, said “this is very strong evidence linking these interstellar shocks to star formation. Understanding this link will help us to develop our theories of star formation”.

The results use observations by the SPIRE and PACS instruments on board Herschel, and are focused on three regions ranging from 500 to 1500 light years away, in the constellations of Cygnus and Aquila, and near Polaris in Ursa Minor. They are part of the Gould Belt, a ring of similar star-forming regions stretched around the sky which is being studied by Herschel as part of the Gould Belt Survey, led by Dr Philippe André from CEA France. "Filaments are the first structures to develop during the fragmentation of interstellar clouds, hence they're the objects to watch when investigating the very early stages of stellar formation", explains Dr André.

Prof Matt Griffin, Cardiff University and lead scientist of the SPIRE instrument, says “this is an incredibly interesting result which no one could have predicted. With observations like these, Herschel is helping us to answer some of the biggest questions which remain in astronomy”.