  Observations of PMS star rotation periods reveal slow rotators in young
clusters of various ages, indicating that angular momentum is somehow removed
from these rotating masses. The mechanism by which spin-up is regulated as
young stars contract has been one of the longest-standing problems in star
formation. Attempts to observationally confirm the prevailing theory that
magnetic interaction between the star and its circumstellar disk regulates
these rotation periods have produced mixed results. In this paper, we use the
unprecedented disk identification capability of the Spitzer Space Telescope to
test the star-disk interaction paradigm in two young clusters, NGC 2264 and the
Orion Nebula Cluster (ONC). We show that once mass effects and sensitivity
biases are removed, a clear increase in the disk fraction with period can be
observed in both clusters across the entire period range populated by cluster
members. We also show that the long-period peak (P $\sim$8 days) of the bimodal
distribution observed for high-mass stars in the ONC is dominated by a
population of stars possessing a disk, while the short-period peak (P $\sim$2
days) is dominated by a population of stars without a disk. Our results
represent the strongest evidence to date that star-disk interaction regulates
the angular momentum of these young stars. This study will make possible
quantitative comparisons between the observed period distributions of stars
with and without a disk and numerical models of the angular momentum evolution
of young stars.

Astrophysics