2001ApJ...554L.197H

We have determined new rotation periods for 404 stars in the Orion Nebula cluster (ONC) using the Wide Field Imager attached to the MPG/ESO 2.2 m telescope on La Silla, Chile. Mass estimates are available for 335 of these, and most have M<0.3 M_☉. We confirm the existence of a bimodal period distribution for the higher mass stars in our sample and show that the median rotation rate decreases with increasing mass for stars in the range 0.1 M_☉<M<0.4 M_☉. While the spread in angular momentum J at any given mass is more than a factor of 10, the majority of lower mass stars in the ONC rotate at rates approaching 30% of their critical breakup velocity, as opposed to 5%-10% for solar-like stars. This is a consequence of both a small increase in observed specific angular momentum (j=J/M) and a larger decrease in the critical value of j with decreasing mass. Perhaps the most striking fact, however, is that j varies by so little–less than a factor of 2–over the interval 0.1-1.0 M_☉. The distribution of rotation rates with mass in the ONC (with an age of ∼1 Myr) is similar in nature to what is found in the Pleiades (with an age of ∼100 Myr). These observations provide a significant new guide and test for models of stellar angular momentum evolution during the protostellar and pre-main-sequence phases.