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W. Herbst (Wesleyan U.), K. Rhode (Yale U.), L. A. Hillenbrand (Cal Tech)
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 1998 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; AJ 11, 283; CH). Here we update those results and provide further analysis based on the extensive optical and infrared study of the cluster by Hillenbrand (1997, AJ 113, 1733) and Hillenbrand et al. (1998, ApJ 116, 1816). Rotation periods with false alarm probablities of less that 1% are now available for 132 members of the ONC. Of these, 62 were detected at multiple epochs with identical periods and an additional 15 were confirmed by Stassun et al. (1999, preprint) in their study of Ori OBI. Therefore, we have a sample of 77 stars with virtually certain rotation periods and another 45 with highly probable periods, all of which are undoubtedly ONC members. Preliminary analysis of this sample reveals the following: 1) the bimodal period distribution for the ONC reported by CH is confirmed both qualitatively and statistically, 2) there is a clear dependence of rotation period on mass, indicating that many stars with M < 0.25 solar masses have not had time to bridge the ``gap" in the period distribution, 3) both excess I-K emission and CaII equivalent widths as measured by Hillenbrand et al. (1998) show weak positive correlations with rotation period among stars with M > 0.25 solar masses, in support of disk interaction models of rotational evolution. The implications of our data for understanding rotational evolution of low mass stars will be discussed. This work was supported by NASA grants from the Origins of Solar Systems program to WH and LAH.
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