Sonic booms give birth to stars

CARDIFF U. (UK) — Interstellar sonic booms traveling through the galaxy leave filaments of gas and dust in their wake that stretch for tens of light years and contain newly-born stars in their densest regions.

Scientists using the the European Space Agency’s Herschel Space Observatory observed one such filament in the constellation of Aquila that contains about 100 infant stars.

The extremely cold gas and dust in interstellar space can only be observed with far-infrared light. While they have been seen before, Herschel’s high resolution 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,” says Doris Arzoumanian, from CEA and the paper’s lead author. “These findings highlight that something must be going on at this particular scale.”

For comparison, the width is around 20,000 times the distance between the Earth and the Sun, or around 1/12th of the distance to the nearest star..

Comparisons made with computer simulations have led the astronomers to conclude that the filaments may be formed when slow shockwaves dissipate in the interstellar clouds.

The shockwaves that travel through the Galaxy, sweeping up gas and dust and forming dense filaments, are the result of the energy produced by exploding stars, which cause a great deal of turbulence in the surrounding regions.

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.

“This is very strong evidence linking these interstellar shocks to star formation,” says Derek Ward-Thompson, professor of physics and astronomy at Cardiff University. “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 Philippe André from CEA France.

A network of filaments in the constellation of Ursa Minor. No stars are forming in this region, that is about 500 light years away.

“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,” says André.

“This is an incredibly interesting result which no one could have predicted,” says Matt Griffin, professor of physics and astronomy at Cardiff. “With observations like these, Herschel is helping us to answer some of the biggest questions which remain in astronomy.”

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