Angular momentum regulation in low-mass young stars surrounded by accretion disks

This contribution examines the role played by circumstellar accretion disks in establishing the initial angular momentum in low-mass pre-main-sequence stars. From study of a sample of 34 T Tauri stars with photometrically derived rotation periods and spectral types later than K.5, we find that the observed periods appear to be related to the presence or absence of an accretion disk. Those stars which we infer to be surrounded by accretion disks have rotation periods P_rot > 4 days with a most probable period P_rot∼ 8.5 days, while those stars which lack accretion disk signatures cover a wide range of rotation periods, from 1.5 < P_rot < 16 days, including a significant number of objects with P_rot < 4 days. This suggests the possibility that the "initial" angular momentum of a star is not established until it dissipates its circumstellar accretion disk. During the disk accretion phase, the stellar angular velocity appears to be regulated at a low value, countering the tendency of the star to spin up both from contraction toward the main sequence and from the accretion of inner disk material of high specific angular momentum. When the accretion disk is dissipated, this regulation mechanism will cease to function. At this point, the star is no longer maintained at a low angular velocity, but is "free" to conserve its angular momentum, and thus to increase its angular velocity in response to contraction and changes in moment of inertia. This hypothesis, combined with a spread in disk dispersal time scales, provides a context for explaining the observed distribution of stellar rotational velocities for stars on the zero-age main sequence in young clusters.