One of the beauties of the Spitzer Space Telescope is that it can pinpoint the swirling dust disks around distant stars. Such dust, heated by the star, puts out an infrared signature that Spitzer can analyze to a degree hitherto unattainable. Now a team of astronomers has observed some 500 young T Tauri stars in the star-forming regions of the Orion nebula. They’ve been looking at how young stars spin, and the effects that dusty disks have on slowing their rotation.
T Tauri stars are ideal for this kind of work. They’re young objects (less than 10 million years old) that are still in the process of gravitational contraction. Such stars often show large accretion disks, but a variant called weak-lined T Tauri stars have little or no disk. Figuring out the various phases of T Tauri formation and how they relate to planets is thus a substantial challenge.
The answers Spitzer has provided are intriguing even if they leave many questions unanswered. Slow-spinning stars are five times more likely to have dust disks than fast-spinning ones. Which implies that the disks, the early construction zones for planets, have a role in slowing the star. But other factors also appear to be involved, including stellar winds. “We can now say that disks play some kind of role in slowing down stars in at least one region, but there could be a host of other factors operating in tandem. And stars might behave differently in different environments,” said Luisa Rebull (Spitzer Science Center, Pasadena).
Image: How does a disk put the brakes on its star? It is thought to yank on the star’s magnetic fields (green lines). When a star’s magnetic fields pass through a disk, they are thought to get bogged down like a spoon in molasses. This locks a star’s rotation to the slower-turning disk, so the star, while continuing to shrink, does not spin faster. Credit: NASA/JPL-Caltech/R. Hurt (SSC).
What we’re aiming for is an understanding of how a star’s rotation rate factors into the formation of planetary systems around it. And things aren’t nearly as clearcut as they might seem — nobody is arguing that fast-spinning stars can’t develop planets. Indeed, Rebull says that a slow spinner may simply take more time than other stars to clear out its disk and begin planet formation.
The exoplanets we’ve learned about so far all circle slowly turning stars; our own Sun rotates once every 28 days, a relatively sedate pace. Finding planets around stars that rotate more quickly is thus key to understanding how rotation and planet formation are intertwined in young stars. That leaves plenty of work for the next generation of space and ground-based telescopes.
The paper is Rebull, Stauffer, Megeath et al., “A Correlation between Pre–Main-Sequence Stellar Rotation Rates and IRAC Excesses in Orion,” Astrophysical Journal 646 (20 July 2006), pp. 297 ff., available online.