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 (PMS) and recently arrived main-sequence stars of solar-like mass (0.4-1.2 M☉) in five nearby young clusters: the Orion Nebula cluster (ONC), NGC 2264, α 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 Myr. There are sufficient data that rotational evolution can now be discussed statistically on the basis of the evolution of these distributions, not just on the evolution of means or ranges, as has been necessary in the past. Our main result is illustrated in Figure 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 Myr, 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 Myr, while the slow rotators lose about two-thirds of theirs. A detectable part of this loss occurs even during the ~1 Myr 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.