Rotation in the Orion Nebula Cluster

Eighteen small (4 arc-min square) fields within the Orion Nebula Cluster (ONC) have 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 Observatory 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 three years of operation of this program were summarized and analyzed by Choi & Herbst (1996). Here we provide an update based on six additional years of observation and the extensive optical and infrared study of the cluster by Hillenbrand (1997) and Hillenbrand et al. (1998). 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 confirmed by Stassun et al. (1999) in their study of Ori OBIc/d. Therefore, we have a sample of 82 stars with virtually certain rotation periods and another 52 with highly probable periods, all of which are cluster members. The bimodal period distribution for the ONC reported by CH is confirmed but we also find a clear dependence of rotation period on mass. This phenomenon can be understood as an effect of deuterium burning, which temporarily slows the contraction and, therefore, spin-up of stars with M ≤ 0.25 M and ages ∼ 1 My. 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 emission, as well as CaII infrared triplet equivalent widths (Hillenbrand et al. 1998), show weak but significant correlations with 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.