  Rotation periods are now available for ~500 pre-main sequence and recently
arrived main sequence stars of solar-like mass (0.4-1.2 M_sun) in five nearby
young clusters: the Orion Nebula Cluster, NGC 2264, alpha Per, IC 2602 and the
Pleiades. In combination with estimates of stellar radii these data allow us to
construct distributions of surface angular momentum per unit mass at three
different epochs: nominally, 1, 2 and 50 My. Our main result is illustrated in
Fig. 18 and may be summarized as follows: (1) 50-60% of the stars on convective
tracks in this mass range are released from any locking mechanism very early on
and are free to conserve angular momentum throughout most of their PMS
evolution, i.e. to spin up and account for the rapidly rotating young main
sequence stars. (2) The other 40-50% lose substantial amounts of angular
momentum during the first few million years, and end up as slowly rotating main
sequence stars. The duration of the rapid angular momentum loss phase is ~5-6
My, which is roughly consistent with the lifetimes of disks estimated from
infrared surveys of young clusters. The rapid rotators of Orion age lose less
than 10% of their (surface) specific angular momentum during the next 50 My
while the slow rotators lose about two-thirds of theirs. A detectable part of
this loss occurs even during the ~1 My interval between the ONC and NGC 2264.
The data support the view that interaction between an accretion disk and star
is the primary mechanism for evolving the broad, bimodal distribution of
rotation rates seen for solar-like stars in the ONC into the even broader
distributions seen in the young MS clusters.
