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W. Huang (Georgia State University)
Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Measurements of the projected rotational velocities for large numbers of cluster stars are becoming practical with the advent of multi-object spectroscopy, and such data make it possible to study the time evolution of the rotational distributions of massive stars. Analysis of these rotational distributions provide information on the angular momentum distribution in newborn clusters, the rotational spin-down due to wind loss, magnetic fields, and binary companions, and the incidence of spin-up due to mass transfer in close binaries. Here I present a survey of rotational velocity measurements of about 500 member stars in 19 galactic clusters in both the northern and southern sky that span an age range of 3 -- 60 Myr. The spectra were obtained with the Hydra multi-object spectrographs at the WIYN 3.5-m and CTIO 4-m telescopes, and they cover the spectral range of 4000 -- 5000 Å~with a resolving power of \lambda / \Delta\lambda = 4000. We constructed a grid of model rotationally-broadened line profiles (for He~I \lambda 4026, He~I \lambda 4387, and He~I \lambda 4471) characterized by spectral type and projected rotational velocity (V\sin i). The effects of aspherical shape and gravitational darkening are considered in these calculations. The V\sin i values of 496 member stars were then determined by fitting the observations with model templates. The changes of the rotational velocity distribution with cluster age are reviewed based upon these V\sin i measurements.
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.