The Mass Dependence of Stellar Rotation in the Orion Nebula Cluster

We have determined new rotation periods for 404 stars in the Orion NebulaCluster using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope onLa Silla, Chile. Mass estimates are available for 335 of these and most have M< 0.3 M_sun. We confirm the existence of a bimodal period distribution for thehigher mass stars in our sample and show that the median rotation ratedecreases with increasing mass for stars in the range 0.1 < M <0.4 M_sun. Whilethe spread in angular momentum (J) at any given mass is more than a factor of10, the majority of lower mass stars in the ONC rotate at rates approaching 30%of their critical break-up velocity, as opposed to 5-10% for solar-like stars.This is a consequence of both a small increase in observed specific angularmomentum (j=J/M) and a larger decrease in the critical value of j withdecreasing mass. Perhaps the most striking fact, however, is that j varies byso little - less than a factor of two - over the interval 0.1-1.0 M_sun. Thedistribution of rotation rates with mass in the ONC (age ~ 1 My) is similar innature to what is found in the Pleiades (age ~ 100 My). These observationsprovide a significant new guide and test for models of stellar angular momentumevolution during the proto-stellar and pre-main sequence phases.Comment: 11 pages, 3 figure