[Previous] | [Session 54] | [Next]
W. Herbst (Wesleyan U.)
There is nothing we know more accurately about a substantial number of pre-main sequence stars than their periods of rotation (P). Photometric monitoring programs have resulted in angular velocity (\omega={{2 \pi} \over P}) measurements, typically accurate to about 1%, for more than a thousand pre-main sequence stars in Orion, NGC 2264, Tau/Aur and other young clusters and associations. Inferred masses and ages for these stars are typically 0.1 - 1 M\odot and 0.5-3 My. The range in \omega for stars of similar mass within the same cluster is about a factor of 20 and there is no apparent dependence on luminosity, radius or age. In the Orion Nebula Cluster there is a striking dependence of rotation on mass (M). Higher mass stars (0.25-1 M\odot) have a bimodal period distribution with peaks near 2 and 8 days, while lower mass stars (0.1-0.25 M\odot) have a unimodal distribution with a peak around 2 days. A similar result (with an interesting difference) has recently been obtained for NGC 2264 by Lamm et al. and is reported elsewhere at this meeting. Evidence is now strongly in favor of disk-locking as the primary mechanism for keeping many stars rotating at \le5% of their critical velocity while others spin much faster. In the ONC, the disk-locking time scale must be ~1 My or less for the solar-like (0.25-1 M\odot ) stars to account for the observed frequency distribution of periods. Since specific angular momentum, j=J/M=k2R2\omega, depends on the stellar radius (R), which is poorly constrained for pre-main sequence stars, we actually know less about angular momentum evolution than one might have expected given the wealth of information on \omega. Spectroscopic measurements of projected equatorial velocities (v sin i) can be useful in a statistical sense in addressing this problem. I will review recent results obtained by this approach. Other issues that will be addressed include cluster-to-cluster differences in rotation properties and the possible effect of environment on rotation. The support of NASA, through its Origins program, for this research is gratefully acknowledged.
[Previous] | [Session 54] | [Next]
Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.