Periodic variability of pre-main-sequence stars in the NGC 2264 OB association

We summarize the results of an observing campaign to identify periodic pre-main-sequence (PMS) variables in the NGC 2264 OB association. We find 201 periodic candidates. The mean age of the NGC 2264 periodic variables is approximately 30% greater than their counterparts in the Orion Nebula cluster. Given the difference in mean age between NGC 2264 and Orion, we expect the typical periodic variable in NGC 2264 to have a period shorter by a factor of similar to 1.6, if the initial distribution of periods in the two clusters is identical and if stellar angular momentum is conserved. However, we find a period distribution indistinguishable from that found in Orion. This suggests that the angular momenta of a significant fraction of PMS stars in the age range similar to 0.4 to similar to 4 Myr must be regulated. To examine the hypothesis that disk locking regulates PMS star angular momenta, we use four different disk indicators (U - V, I-C - K-s, H - K-s, and Halpha) to search for correlations between period and disk indicator. We find no conclusive evidence that more slowly rotating stars have disk indicators, or that faster rotating stars are less likely to have disk indicators.