Disk-locking Regulates Stellar Rotation in Young Clusters: Insights from NGC 2264

Many young clusters possess extended main sequences, a phenomenon commonly ascribed to stellar rotation. However, the mechanism behind their very wide stellar rotation distributions remains unclear. A proposed explanation is that magnetic star-disk interaction can regulate stellar rotation, i.e., protostars with longer disk lifetimes will eventually evolve into slow rotators, and vice versa. To examine this hypothesis, we took the star-forming region NGC 2264, as a test bed. We have studied its high-mass pre-main-sequence and zero-age main-sequence (ZAMS) stars. We find that, on average, diskless pre-main-sequence stars rotate faster than their disk-bearing counterparts. The stellar rotation distribution of the ZAMS stars is similar to evolved young clusters. We conclude that disk-locking may play a crucial role in the rotational velocity distribution of intermediate-mass early-type stars. We suggest that the observed wide stellar rotation distributions in many young clusters can occur in their early stages.