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Shining light on
dark stellar cocoons

Posted: 29 September 2010

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By studying light that has bounced off dark stellar dust clouds, astronomers are gleaning new information on the earliest phases of star formation.

The dark core of molecular cloud L183. The image on the far right shows the core as seen at longer wavelengths of infrared light (8 microns) where the core appears dark. The middle image shows the core as seen at a shorter infrared wavelength (3.6 microns) where it is light up because it is deflecting starlight from nearby stars. The image on the left is a combination of the other two images. Image credit: NASA/JPL-Caltech/Observatoire de Paris/CNRS.

The team of astronomers used NASA's Spitzer Space Telescope to study the light deflected off the dense clouds of gas and dust that are completely opaque in ordinary light. "Dark clouds in our Milky Way Galaxy, far from Earth, are huge places where new stars are born," says Laurent Pagani of the Observatoire de Paris and the Centre National de la Recherche Scientifique. "But they are shy and hide themselves in a shroud of dust so that we cannot see what happens inside. Now we have found a new way to peer into them. They are like ghosts because we see them but we also see through them."

The phenomenon of the reflected light has been nicknamed 'coreshine', which the team first identified in the molecular cloud L183 in the constellation Serpens Cauda last year. Since the way in which the light is scattered is dependent on the size of the dust grains, the team translated the result into dust particles some 1 micrometre across, instead of the 0.1 microns previously thought. Smaller particles would not be big enough to scatter the light.

Thinking it might be a chance result, the team followed up with the study of 110 more cores, finding half of them to exhibit the same 'coreshine' phenomenon. The implication of the larger grain size means that planets will likely coalesce more quickly, and perhaps when the star is still in its pre-embryonic phase. The nature of the deflected starlight is also dependent on the structure of the core in terms of the distribution of gas and dust, so astronomers can begin to construct three-dimensional models of these star-forming pods. The coreshine will also shed light on the age of the cores – the more developed cores will have large dust grains.

"We're opening a new window on the realm of dark, star-forming cores," adds Pagani.