Rotational evolution of low mass stars: The case of NGC 2264

Abstract. Our study is based on an extensive photometric monitoring program in the young (2–4 Myr) open cluster NGC 2264 by Lamm et al. (2004a). This program resulted in a sample of 405 periodic variables which are most likely pre-main sequence (PMS) members of the cluster. The periodic variability of these stars results from the rotational modulation of the light by stellar spots. In this paper we investigate the rotation period evolution of young stars. This is done by comparing the period distribution of the older NGC 2264 with that of the younger Orion Nebular Cluster (ONC, age: ∼ 1 Myr) which is known from the literature. The age ratio between the two clusters was estimated on the basis of PMS models to be about 2 +0.75. We find −0.5 that the period distribution of NGC 2264 is similar in form to the ONC but shifted to shorter periods. In both clusters the period distribution depends strongly on the mass and it is bimodal for higher mass stars with M> ∼ 0.25 M ⊙ while it is unimodal for lower mass stars with M < ∼ 0.25 M⊙. In addition the lower mass stars rotate much faster on average than the higher mass stars. Quantitative comparison between the period distributions of both clusters suggests that a large fraction (about 80%) of stars have spun up from the age of the ONC to the age of NGC 2264. Based on this estimate and the estimated age ratio between the two clusters we find that the average spin up by a factor of 1.5 – 1.8 from the age of the ONC to the age of NGC 2264 is consistent with a decreasing stellar radius and conservation of angular momentum, for most stars. However, within NGC 2264 we did not find any significant spin up from the younger to older stars in the cluster. We also found indications for some ongoin