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-mai n-sequence stars. From study of a sample of 34 T Tauri stars with phot ometrically derived rotation periods and spectral types later than K5, we find that the observed periods appear to be related to the presenc e or absence of an accretion disk. Those stars which we infer to be su rrounded by accretion disks have rotation periods P(rot) > 4 days with a most probable period P(rot) approximately 8.5 days, while those sta rs 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 t he ''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 contra ction toward the main sequence and from the accretion of inner disk ma terial of high specific angular momentum. When the accretion disk is d issipated, this regulation mechanism will cease to function. At this p oint, the star is no longer maintained at a low angular velocity, but is ''free'' to conserve its angular momentum, and thus to increase its angular velocity in response to contraction and changes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 tim e scales, provides a context for explaining the observed distribution of stellar rotational velocities for stars on the zero-age main sequen ce in young clusters.