Additional Periodic Variables in NGC 2264

We present results from a second season of monitoring elds in the young cluster NGC 2264 at Van Vleck Observatory. In Paper I of this series, we reported the discovery of nine periodic variablesall interpreted as spotted premain-sequence stars. That has now been increased to 31, one of which is quite unusual. The preliminary result reported in Paper Ithat the frequency distribution of rotation periods in NGC 2264 is signicantly di erent from what is seen in the Orion Nebula cluster (ONC)is con-rmed. In particular, the distribution peaks at a period of about 4 days, precisely where there is a gap in the ONC distribution. There is also a good number of very rapidly rotating stars and a long tail of slow rotators. The distribution can be understood, at least qualitatively, in terms of a disk-locking model of rotational evolution. In this interpretation, the majority of stars in NGC 2264 must have unlocked from their disks by an age of about 1 million years, and spun up roughly in accordance with conservation of angular momentum. A minority (about 20%) are conserving angular velocity instead (i.e., locked to their disks). The unusual star, 15D, has an apparent period of about 48 days and a light curve indicative of an eclipse by a nonstellar object. Its amplitude exceeds 3 mag. Our fragmentary data suggest a complex structure for the eclipsing body. It could be a feature (protoplanet ?) in this stars circumstellar disk and, as such, deserves observational attention.

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