Effects of Metallicity on the Rotational Velocities of Massive Stars

Recent theoretical predictions for low metallicity massive stars predict thatthese stars should have drastically reduced equatorial winds (mass loss) whileon the main sequence, and as such should retain most of their angular momentum.Observations of both the Be/(B+Be) ratio and the blue-to-red supergiant ratioappear to have a metallicity dependence that may be caused by high rotationalvelocities. We have analyzed 39 archival Hubble Space Telescope ImagingSpectrograph (STIS), high resolution, ultraviolet spectra of O-type stars inthe Magellanic Clouds to determine their projected rotational velocities V sini. Our methodology is based on a previous study of the projected rotationalvelocities of Galactic O-type stars using International Ultraviolet Explorer(IUE) Short Wavelength Prime (SWP) Camera high dispersion spectra, whichresulted in a catalog of V sin i values for 177 O stars. Here we presentcomplementary V sin i values for 21 Large Magellanic Cloud and 22 SmallMagellanic Cloud O-type stars based on STIS and IUE UV spectroscopy. Thedistribution of V sin i values for O type stars in the Magellanic Clouds iscompared to that of Galactic O type stars. Despite the theoretical predictionsand indirect observational evidence for high rotation, the O type stars in theMagellanic Clouds do not appear to rotate faster than their Galacticcounterparts.Comment: accepted by ApJ, to appear 20 December 2004 editio