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Herschel links star formation to Sonic Booms

19 April 2011

A team of astronomers, including members of Cardiff University’s School of Physics and Astronomy, have used the European Space Agency's Herschel Space Observatory to show 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.

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.

Professor Derek Ward-Thompson from Cardiff University's School of Physics and Astronomy, 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.

Professor Matt Griffin from Cardiff University's School of Physics and Astronomy and lead scientist of the SPIRE instrument, said: "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."


"Characterizing interstellar filaments with Herschel in IC5146" by D. Arzoumanian et al. was published on 13 April 2011 online by Astronomy and Astrophysics. The research includes astronomers from Cardiff University, the Open University and the Rutherford Appleton Laboratory.

Gould Belt Survey
This work is based on observations performed within the Herschel Key Programme "Probing the origin of the stellar initial mass function: A wide-field Herschel photometric survey of nearby star-forming cloud complexes" which aims at mapping large portions of the Gould Belt with the SPIRE and PACS instruments on Herschel. The survey will cover the wavelength range 70–500 ìm in order to study the demographics of star formation and the details of its onset. When completed, the entire survey will comprise about 20 star-forming regions.

The first three regions to be analysed in detail are: the Aquila Rift, a very active star-forming complex at a distance of about 850 light years; the Polaris Flare, a translucent cloud with little to no star formation at a distance of about 490 light years; IC 5146, a star-forming cloud at a distance of about 1500 light years.

The Aquila and Polaris fields were observed during the Science Demonstration Phase at the end of 2009; IC 5146 was observed on 29 May 2010. All three fields were observed in the parallel scan mode of Herschel with both SPIRE and PACS. The SPIRE observations were obtained by the Star Formation group of the SPIRE consortium.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

The SPIRE instrument contains an imaging photometer (camera) and an imaging spectrometer. The camera operates in three wavelength bands centred on 250, 350 and 500 ìm, and so can make images of the sky simultaneously in three sub-millimetre colours. SPIRE has been developed by a consortium of institutes led by Cardiff Univ. (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA).

PACS is also an imaging photometer (camera) and an imaging spectrometer. The camera operates in three bands centred on 70, 100, and 160 ìm, respectively. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA- PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy), and CICT/MCT (Spain).

Further information:

Professor Matt Griffin
School of Physics and Astronomy, Cardiff University
Tel: +44 (0)2920 874 203

Professor Derek Ward-Thompson
School of Physics and Astronomy, Cardiff University
Tel: +44 (0) 2920 875 314

Dr Chris North
UK Herschel Outreach Officer
Cardiff University
Tel: +44 (0)2920 870 537