Astronomers have finally found evidence that the rotation of young, whirling stars is slowed down by the planet-forming disks surrounding them. New data from NASA¿s Spitzer Space Telescope support the hypothesis that protoplanetary disks absorb some of a young star¿s angular momentum, which, if left unchecked, would cause the star to spin so fast it would fly apart under its own inertia. Scientists have believed for decades that planetary disks were probably responsible for slowing down spinning stars but until now observations did not clearly support the idea.

¿We had a really nice theoretical model in the nineties and some of the data seemed to support it, but the more data we got, the muddier the waters became,¿ says Luisa Rebull of NASA's Spitzer Science Center in Pasadena, Calif., lead author of the paper that appeared in The Astrophysical Journal July 20. The model was based on the fact that as a large cloud of gas and debris begins contracting under its own gravity, conservation of angular momentum dictates that it will rotate faster and faster as it shrinks, much like a spinning skater who pulls in her arms in order to twirl at top speed. These infant stars eventually spin so fast that any excess gas and dust is flattened into a pancakelike disk around the star, which may eventually yield planets. Although all stars are thought to form these protoplanetary disks, for unknown reasons some disks remain prominent and others become smaller or nearly disappear. Because a disk is composed of ionized gas and dust, a spinning star¿s magnetic field gets caught up in it and the result is ¿like a spoon being dragged through molasses,¿ Rebull says. Angular momentum is transferred to the disk and the star rotates more slowly, with a much greater effect seen in stars with larger disks.

¿It was too good a solution to let go,¿ says Rebull, who has been working on this problem for over a decade. Until now, observations have yielded mixed results--only some of the data supported the theory--and all the observations suffered from small sample sizes and the inaccuracies inherent in using optical telescopes to determine which young stars had prominent disks. Finally, the launch of Spitzer in August 2003 meant astronomers could readily identify these large disks; they are heated by the stars and give off infrared light in the frequencies Spitzer is designed to detect. Rebull examined nearly 500 stars in the Orion Nebula and found that slowly spinning stars were five times more likely to have prominent disks than fast spinners. ¿This is the clearest evidence yet that indeed the disks are where the angular momentum is going,¿ says Rebull. ¿It¿s very encouraging that we¿re on the right track.¿

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