Rotation in the Orion Nebula cluster

Eighteen small (4' square) fields within the Orion Nebula cluster (ONC) hav e been photometrically monitored for one or more observing seasons between 1990 and 1999 with a CCD attached to the 0.6 m telescope at Van Vleck Obser vatory on the campus of Wesleyan University. Data were obtained exclusively in the Cousins I band on between 25 and 40 nights per season. Results from the first 3 yr of operation of this program were summarized and analyzed b y Choi & Herbst. Here we provide an update based on an additional 6 yr of o bservation and the extensive optical and infrared study of the cluster by H illenbrand et al. Rotation periods with false-alarm probabilities FAP < 1% are now available for 134 members of the ONC. Of these, 67 were detected at multiple epochs with identical periods by us, and an additional 15 were co nfirmed by Stassun et al. in their study of Ori OB1c and OB1d Therefore, we have a sample of 82 stars with virtually certain rotation periods and anot her 52 with highly probable periods, all of which are cluster members. The bimodal period distribution for the ONC reported by Choi & Herbst is confir med, but we also find a clear dependence of rotation period on mass. This p henomenon can be understood as an effect of deuterium burning, which tempor arily slows the contraction and, therefore, spin-up of stars with M less th an or equal to 0.25 M . and ages similar to 1 Myr. Stars with M < 0.25 M . have not had time to bridge the gap in the period distribution at around 4 days. Excess H-K and I-K emissions, as well as Ca II infrared triplet equiv alent widths (Hillenbrand et al.), show weak but significant correlations w ith rotation period among stars with M > 0.25 M . . Our results provide new observational support for the importance of disks in the early rotational evolution of low-mass stars.