The Physical Properties and Effective Temperature Scale of O-type Stars as a Function of Metallicity. I. A Sample of 20 Stars in the Magellanic Clouds

We have obtained HST and ground-based observations of a sample of 20 O-type stars in the LMC and SMC, including six of the hottest massive stars known (subtypes O2-3) in the R136 cluster. In general, these data include (a) the HST UV spectra in order to measure the terminal velocities of the stellar winds, (b) high signal-to-noise, blue-optical data where the primary temperatureand gravity-sensitive photospheric lines are found, and (c) nebular-free Hα profiles, which provide the mass-loss rates. We find that the older (FOS) HST data of the R136 stars (which were obtained without the benefits of sky measurements) Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 6417, 7739, 8633, and 9412. This paper also draws heavily from data obtained from the data archive at STScI. 2 Visiting astronomer, Cerro Tololo Inter-American Observatory (CTIO), a division of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.