Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples withwell-defined ages and initial chemical composition. Here we present a survey ofprojected rotational velocities for a large sample of mainly B-type stars inyoung clusters to study the time evolution of the rotational properties ofmassive stars. The survey is based upon moderate resolution spectra made withthe WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs,and the target stars are members of 19 young open clusters with an age range ofapproximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387,4471 and Mg II 4481 using model theoretical profiles to find projectedrotational velocities for a total of 496 OB stars. We find that there are fewerslow rotators among the cluster B-type stars relative to nearby B stars in thefield. We present evidence consistent with the idea that the more massive Bstars (M > 9 solar masses) spin down during their main sequence phase. However,we also find that the rotational velocity distribution appears to show anincrease in the numbers of rapid rotators among clusters with ages of 10 Myrand higher. These rapid rotators appear to be distributed between the zero ageand terminal age main sequence locations in the Hertzsprung-Russell diagram,and thus only a minority of them can be explained as the result of a spin up atthe terminal age main sequence due to core contraction. We suggest instead thatsome of these rapid rotators may have been spun up through mass transfer inclose binary systems.Comment: 33 pages, 11 figures, accepted by Ap