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 Hubble Space Telescope (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-O3) in the R136 cluster. In general, these data include (1) the HST UV spectra in order to measure the terminal velocities of the stellar winds, (2) high signal-to-noise, blue-optical data where the primary temperature- and gravity-sensitive photospheric lines are found, and (3) nebular-free Hα profiles, which provide the mass-loss rates. We find that the older (Faint Object Spectrograph) HST data of the R136 stars (which were obtained without the benefits of sky measurements) suffered from significant nebular emission, which would increase the derived mass-loss rates by factors of ~3, all other factors being equal. We also find several stars in the SMC for which the N III λλ4634, 4642 and He II λ4686 emission "f" characteristics do not appear to follow the same pattern as in Galactic stars. Since He II emission is due to the stellar wind (which will be weaker in SMC for stars of the same luminosity), while N III emission is a complex non-LTE (NLTE) effect affected mostly by temperature, it would not be surprising to find that these features do not correlate with each other or with luminosity in SMC stars in the same was as they do in Galactic stars, but theory does not provide a clean answer, and analysis of more stars (both SMC and Galactic) is needed to resolve this issue. The line-blanketed NLTE atmosphere code FASTWIND was then used to determine the physical parameters of this sample of stars. We find good agreement between the synthetic line profiles for the hydrogen, He I, and He II lines in the majority of the stars we analyzed; the three exceptions show evidence of being incipiently resolved spectroscopic binaries or otherwise spectral composites. One such system is apparently an O3 V+O3 V eclipsing binary, and a follow-up radial velocity study is planned to obtain Keplerian masses. Although we did not use them to constrain the fits, good agreement is also found for the He I λ3187 and He II λ3203 lines in the near-UV, which we plan to exploit in future studies. Our effective temperatures are compared with those recently obtained by Repolust, Puls & Herrero for a sample of Galactic stars using the same techniques. We find that the Magellanic Cloud sample is 3000-4000 K hotter than their Galactic counterparts for the early through mid-O typess. These higher temperatures are the consequence of a decreased importance of wind emission, wind blanketing, and metal-line blanketing at lower metallicities.