  We have determined new rotation periods for 404 stars in the Orion Nebula
Cluster 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_sun. 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 <0.4 M_sun. 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 break-up 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 two - over the interval 0.1-1.0 M_sun. The
distribution of rotation rates with mass in the ONC (age ~ 1 My) is similar in
nature to what is found in the Pleiades (age ~ 100 My). These observations
provide a significant new guide and test for models of stellar angular momentum
evolution during the proto-stellar and pre-main sequence phases.
