ROTATION PERIODS OF T-TAURI STARS IN THE ORION NEBULA CLUSTER - A BIMODAL FREQUENCY-DISTRIBUTION

CCD photometric monitoring of 11 fields in the Orion Nebula Cluster (ONC) was carried out at Van Vleck Observatory during the 1991/92 Milky Way season. More than 180 variable stars were detected and analysis of their light curves has led to the discovery of 35 rotation periods for pre-main-sequence stars, more than doubling the existing all-sky sample. Our data reveal a distinct bimodality in the frequency distribution of rotation periods of the ONC stars. There is a "rapid rotator" group with a mean period of 2.2 days and a dispersion of about 1 day, and a "slow rotator" group with mean period around 8.5 days and dispersion of about 2.5 days. A comparison is made between the ONC results and the previously known rotation period distribution for stars in T associations, which is also bimodal. The ONC sample has an enhanced long period tail and more long period stars relative to short period stars; both differences could be the result of selection effects influencing the T association sample. The significance of these results for studies of angular momentum evolution in low mass stars is briefly discussed and spectroscopic study of the stars with known rotation periods urged.