Rotational Evolution of Solar‐like Stars in Clusters from Pre–Main Sequence to Main Sequence: Empirical Results

Rotation periods are now available for ~500 pre-main sequence and recentlyarrived main sequence stars of solar-like mass (0.4-1.2 M_sun) in five nearbyyoung clusters: the Orion Nebula Cluster, NGC 2264, alpha Per, IC 2602 and thePleiades. In combination with estimates of stellar radii these data allow us toconstruct distributions of surface angular momentum per unit mass at threedifferent epochs: nominally, 1, 2 and 50 My. Our main result is illustrated inFig. 18 and may be summarized as follows: (1) 50-60% of the stars on convectivetracks in this mass range are released from any locking mechanism very early onand are free to conserve angular momentum throughout most of their PMSevolution, i.e. to spin up and account for the rapidly rotating young mainsequence stars. (2) The other 40-50% lose substantial amounts of angularmomentum during the first few million years, and end up as slowly rotating mainsequence stars. The duration of the rapid angular momentum loss phase is ~5-6My, which is roughly consistent with the lifetimes of disks estimated frominfrared surveys of young clusters. The rapid rotators of Orion age lose lessthan 10% of their (surface) specific angular momentum during the next 50 Mywhile the slow rotators lose about two-thirds of theirs. A detectable part ofthis 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 staris the primary mechanism for evolving the broad, bimodal distribution ofrotation rates seen for solar-like stars in the ONC into the even broaderdistributions seen in the young MS clusters.Comment: Accepted and Scheduled for ApJ 10 November 200