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Interstellar searchlights catch star factories in their beams

April 04, 2002

Jets of particles from newly formed stars are acting like searchlights, piercing the gloom of dark interstellar clouds to pick out clumps of gas that may become future stars. Astronomers at University College London (UCL) and the University of Barcelona have discovered how these interstellar beams mark the clumps with a distinctive chemical signature, which makes them detectable with radio telescopes. These new developments in understanding how and where new stars are created will be discussed by Dr Serena Viti of University College London on Wednesday 10 April in her talk on `Chemistry and Star Formation` at the National Astronomy Meeting in Bristol. By identifying and observing these clumps, Dr Viti and her colleagues will learn more about the earliest precursors of star like our Sun.

Stars and their planets are born in dark molecular clouds in interstellar space. Somehow, gravity pulls together clumps in these clouds and overcomes all the forces that resist it - gas pressure, turbulence, and magnetism. Clouds once thought to be smooth have turned out to be very clumpy, as theoretical work by the research group at UCL predicted some time ago. Some of the clumps are transient, existing for a only million years - a very short time by astronomical standards - before they disperse again. More massive clumps will go on to form stars, because gravity in them is stronger. But astronomers admit that, like the dark clouds themselves, their understanding of this process is foggy. To learn more, they want to identify and study the clumps of gas that will make stars. The question has been , how to find them.

"If you want to find something in the garden on a dark night you use a torch, and if you want to find clumps in a dark interstellar cloud, you can do something rather similar," says Dr Viti. "You can make use of the fact that young stars fire out jets - narrow beams of very fast particles - that can travel for distances of light years through interstellar space until they hit a dark molecular cloud."

Where a jet strikes a molecular cloud, a small region becomes so hot that it shines almost like a star. Blobs of hot, glowing gas created in this way are called Herbig-Haro (or HH) objects, after the two astronomers who first studied them in detail. Radiation from HH objects in places that would otherwise be dark (because there are no stars in the vicinity) causes chemical changes in any nearby gas clump and imprints on the clump a special "signature" by enhancing the abundance of certain molecules.

Radio telescopes can detect these signatures, then reveal the extent of a clump as well as its physical and chemical nature. Dr Viti and her colleagues at UCL predicted theoretically what this special chemical signature should be and, with colleagues from the University of Barcelona, they have used radio telescopes in Hawaii and California to confirm its detection.

"It`s remarkable when you realise that the energy that created the chemical signature of a clump arose in a star a considerable distance from its cloud, and travelled in a collimated beam, perhaps some light years through interstellar space, to create the HH object light source," says Dr Viti. "That`s some searchlight!"

Royal Astronomical Society (RAS)

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