Effects of metallicity on the rotational velocities of massive stars

Recent theoretical predictions for low-metallicity massive stars predict that these stars should have drastically reduced equatorial winds ( mass loss) while on the main sequence, and so should retain most of their angular momentum. Observations of both the Be/(B + Be) ratio and the blue-to-red supergiant ratio appear to have a metallicity dependence that may be caused by high rotational velocities. We have analyzed 39 archival Hubble Space Telescope Imaging Spectrograph (STIS), high-resolution, ultraviolet spectra of O-type stars in the Magellanic Clouds to determine their projected rotational velocities Vsin i. Our methodology is based on a previous study of the projected rotational velocities of Galactic O-type stars using International Ultraviolet Explorer (IUE) short-wavelength prime (SWP) camera high-dispersion spectra, which resulted in a catalog of Vsin i values for 177 O-type stars. Here we present complementary Vsin i values for 21 Large Magellanic Cloud and 22 Small Magellanic Cloud O-type stars based on STIS and IUE UV spectroscopy. The distribution of Vsin i values for O-type stars in the Magellanic Clouds is compared to that of Galactic O-type stars. Despite the theoretical predictions and indirect observational evidence for high rotation, the O-type stars in the Magellanic Clouds do not appear to rotate faster than their Galactic counterparts.