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

Abstract

We have obtained Hubble Space Telescope (HST) and ground-basedobservations 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 theR136 cluster. In general, these data include (1) the HST UV spectra inorder to measure the terminal velocities of the stellar winds, (2) highsignal-to-noise, blue-optical data where the primary temperature- andgravity-sensitive photospheric lines are found, and (3) nebular-freeH{α} profiles, which provide the mass-loss rates. We find that theolder (Faint Object Spectrograph) HST data of the R136 stars (which wereobtained without the benefits of sky measurements) suffered fromsignificant nebular emission, which would increase the derived mass-lossrates by factors of ~3, all other factors being equal. We also findseveral stars in the SMC for which the N III {λ}{λ}4634, 4642and He II {λ}4686 emission ``f'' characteristics do not appear tofollow the same pattern as in Galactic stars. Since He II emission isdue to the stellar wind (which will be weaker in SMC for stars of thesame luminosity), while N III emission is a complex non-LTE (NLTE)effect affected mostly by temperature, it would not be surprising tofind that these features do not correlate with each other or withluminosity 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. Theline-blanketed NLTE atmosphere code FASTWIND was then used to determinethe physical parameters of this sample of stars. We find good agreementbetween the synthetic line profiles for the hydrogen, He I, and He IIlines in the majority of the stars we analyzed; the three exceptionsshow evidence of being incipiently resolved spectroscopic binaries orotherwise spectral composites. One such system is apparently an O3 V+O3V eclipsing binary, and a follow-up radial velocity study is planned toobtain Keplerian masses. Although we did not use them to constrain thefits, good agreement is also found for the He I {λ}3187 and He II{λ}3203 lines in the near-UV, which we plan to exploit in futurestudies. Our effective temperatures are compared with those recentlyobtained by Repolust, Puls {\amp} Herrero for a sample of Galactic starsusing the same techniques. We find that the Magellanic Cloud sample is3000-4000 K hotter than their Galactic counterparts for the earlythrough mid-O typess. These higher temperatures are the consequence of adecreased importance of wind emission, wind blanketing, and metal-lineblanketing at lower metallicities.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555. These observations are associatedwith programs 6417, 7739, 8633, and 9412. This paper also draws heavilyfrom data obtained from the data archive at STScI.