Additional periodic variables in NGC 2264

We present results from a second season of monitoring fields in the young cluster NGC 2264 at Van Vleck Observatory. In Paper I of this series, we reported the discovery of nine periodic variables-all interpreted as spotted pre-main-sequence stars. That has now been increased to 31, one of which is quite unusual. The preliminary result reported in Paper I-that the frequency distribution of rotation periods in NGC 2264 is significantly different from what is seen in the Orion Nebula cluster (ONC)-is confirmed. 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 star's circumstellar disk and, as such, deserves observational attention.