The early angular momentum history of low-mass stars: Evidence for a regulation mechanism

We examine the early angular momentum history of stars in young clusters via 197 photometric periods in fields flanking the Orion Nebula cluster (ONC), 81 photometric periods in NGC 2264, and 202 measurements of v sin i in the ONC itself. We show that pre-main-sequence stars spanning an age range from 0.1 to 3 Myr do not appear to conserve stellar angular momentum as they evolve down their convective tracks, but instead preserve the same range of periods even though they have contracted by about a factor of 3. This result seems to require a mechanism that regulates the angular velocities of young stars. We discuss several candidate mechanisms. The most plausible appears to be disk locking, though most of our stars do not have I-C-K-s excesses suggestive of disks. However, a decisive test of this hypothesis requires a more sensitive diagnostic than the I-C-K-s excesses used here.