Herschel links star formation to sonic booms
The connection between filaments and star formation was once unclear, but now scientists can actually see stars forming like beads on strings.
April 14, 2011
The European Space Agency’s (ESA) Herschel space observatory has revealed that nearby interstellar clouds contain networks of tangled gaseous filaments. Intriguingly, each filament is approximately the same width, hinting that they may result from interstellar sonic booms throughout our galaxy.
Dense filaments of gas in the IC5146 interstellar cloud. This image was taken by ESA’s Herschel space observatory at infrared wavelengths 70, 250, and 500 microns. Stars are forming along these filaments. ESA/Herschel/SPIRE/PACS/D. Arzoumanian (CEA Saclay) for the “Gould Belt survey” Key Program Consortium
The filaments are huge, stretching for tens of light-years through space, and Herschel has shown that newly born stars are often found in the densest parts of them. One filament Herschel imaged in the Aquila region contains a cluster of about 100 infant stars.
Other infrared satellites have glimpsed such filaments in interstellar clouds before, but the images have never been clear enough for scientists to measure the filaments’ widths. Now, Herschel has shown that, regardless of the length or density of a filament, the width is always roughly the same.
"This is a very big surprise," said Doris Arzoumanian from the Laboratoire AIM Paris-Saclay. Together with Philippe André from the same institute and other colleagues, she analyzed 90 filaments and found they were all about 0.3 light-year across, or about 20,000 times the distance of Earth from the Sun. This consistency of the widths demands an explanation.
Comparing the observations with computer models, the astronomers concluded that filaments probably form when slow shock waves dissipate in the interstellar clouds. These shock waves are mildly supersonic, and they are a result of the copious amounts of turbulent energy injected into interstellar space by exploding stars. They travel through the dilute sea of gas found in the galaxy, compressing and sweeping it up into dense filaments as they go.
Interstellar clouds are usually extremely cold, about 10 Kelvin above absolute zero, and this makes the speed of sound in them relatively slow at just 0.1 miles per second (0.2 km/s), as opposed to 0.21 mile per second (0.34 km/s) in Earth's atmosphere at sealevel.
These slow shock waves are the interstellar equivalent of sonic booms. The team suggests that as the sonic booms travel through the clouds, they lose energy, and where they finally dissipate, they leave these filaments of compressed material.
"This is not direct proof, but it is strong evidence for a connection between interstellar turbulence and filaments. It provides a strong constraint on theories of star formation," said André.
The team made the connection by studying three nearby clouds, known as IC 5146, Aquila, and Polaris, using Herschel's SPIRE and PACS instruments.
"The connection between these filaments and star formation used to be unclear, but now thanks to Herschel, we can actually see stars forming like beads on strings in some of these filaments," said Göran Pilbratt from ESA.