AAS 200th meeting, Albuquerque, NM, June 2002
Session 70. Angular Momentum Evolution and Young Stars
Display, Thursday, June 6, 2002, 9:20am-4:00pm, SW Exhibit Hall

## [70.02] Variability and Rotation of Pre-Main Sequence Stars in NGC2264

M. Lamm, R. Mundt, C.A.L. Bailer-Jones (MPI for Astronomy, Heidelberg), W. Herbst (Wesleyan Univ., Middletown)

We present the first results of a variability and rotational period study of the pre-main sequence (PMS) stars in the young (2-4 Myr) open cluster NGC\,2264 (d=770 pc). We have carried out I band observations of this cluster on 44 nights over a total time interval of two months using the Wide Field Imager (WFI) attached to the MPG/ESO 2.2m telescope at La Silla, Chile. For the period determination we make use of the fact that the stellar spots on the surface of these stars cause periodic light intensity modulations. All of the 11000 stars in the field (\begin{math}32'\times 34'\end{math}) with magnitudes between I=9 mag and I=21 mag have been checked for both periodic and irregular variability. Altogether we found about 600 periodic variables and about 200 irregular variable stars. The rotational periods are typically between 0.5 and 10 days with peak-to-peak variations between 0.02 mag and more than 1 mag. The period distribution is magnitude dependent and for stars with \begin{math}I \leq 15.5\end{math} mag (which corresponds roughly to \begin{math}M \geq 0.3 M\odot\end{math}) it is bimodal with peaks at about 1 and 4 days. For stars with \begin{math}I \geq 15.5\end{math} we find an unimodal distribution with a peak at about 1 day. When we compare out results for NGC\,2264 with the younger Orion Nebular cluster (ONC, age: 1 Myr, see Herbst et al., at this meeting) we find that in NGC\,2264 the peaks in both the bimodal distribution of the higher mass stars and in the unimodal distribution of the lower mass stars are at about half of the values measured for the ONC. If one assumes that most stars in NGC\,2264 are no longer magnetically locked to their disks (i.e. their angular momentum is conserved) their much higher rotational speed can simply be interpreted as a result of their larger age and a correspondingly smaller stellar radius.