ROTATION OF YOUNG SOLAR-TYPE STARS IN THE ORION NEBULA REGION

Approximately 50 stars in the Orion Nebula region with V = 12-14, B - V = 0.6-1.2 were observed with the intent of empirically determining the evolution of rotation on the approach to the main sequence. Two fields of about 40' extent were surveyed, Id, centered on the Trapezium region, and Ic1, about 1 degree away. Most of the stars are of 1.0-1.6 M., with apparent ages ranging from 10(6) to a few times 10(7) yr, and all are proper motion members. The observed distribution of v sin i shows a range from less than 15 km s-1 (unresolved) to about 50 km s-1. There is no clear increase from younger to older stars as they approach the main sequence. Rather, a wide range in v sin i is present at all evolutionary phases, and this apparently can be traced back to the conditions of star formation. The data contradict the suggestion that all late-type stars go through a brief phase of extremely rapid rotation when they first arrive on the main sequence. If the observed distribution of v sin i is evolved to the main sequence with moderate angular momentum loss which is not strongly dependent on rotation velocity, the distribution observed among Pleiades K stars and G and K stars of the alpha Perseus cluster is reproduced. The data support the suggestion that angular momentum loss due to stellar winds brakes the convection zone more than the interior, so that solid-body rotation is not maintained. Seven of the Orion stars are identified as possible spectroscopic binaries, needing confirmation by additional observations. Six are in region Id and only one in region Ic1, which may be a manifestation of the higher star density in the former region. The proportion of suspected spectroscopic binaries in region Id is much higher than one would expect to find among field stars using the same observational techniques.