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Session 32 - Stars in Various Throes of Stellar Evolution.
Oral session, Monday, January 13
Piers 2/3,
We have constructed models of low mass stars (1.2-0.5 M_ødot) including the effects of internal angular momentum transport, angular momentum loss, and accretion disk lifetimes in the pre-main-sequence phase of evolution. We have systematically and consistently explored the effects of varying these parameters.
The angular momentum loss is prescribed to occur through a magnetic wind and to saturate at some value of the rotation. The resulting rotation rates are found to be a strong function of the value of this saturation threshold. We compare rotation data from open clusters with our models. We find that a mass dependent saturation threshold is necessary to reproduce the distribution of rotation rates seen in open clusters. The inferred lifetime of the circumstellar accretion disk is found to depend on the assumptions made about core-envelope decoupling. The solid body models require longer disk lifetimes (\sim 10-20 Myr) while the differential rotation models require shorter disk lifetimes (\sim 3-10 Myr). The latter lifetimes are consistent with the duration of the infra-red excess observed in pre-main-sequence stars, believed to be a signature of accretion disks. Our models predict an increase in rotation rate at older ages for decreased mass as is seen from recent data obtained in the Pleiades and Hyades.
We also present the results of a monitoring program to determine the photometric rotational periods of low mass stars in the Pleiades. There is now a significant sample in the Pleiades which has both projected velocity measurements (v \sin i) and photometric rotational period measurements and we discuss this sample. We discuss the correlation between X-ray activity and rotation rates. Consistent with earlier work, we find that there is a correlation between the inverse Rossby number (ratio of convective overturn time to the rotation period) and the X-ray activity, which implies that the saturation is mass dependent.
