Stellar winds and UV extinction of OB stars in M 33 and M 31.

Abstract

We report on the continued study of the most luminous hot stars in galaxies of the Local Group, with the aim of studying their spectroscopic and wind characteristics, and comparing them with similar Milky Way stars. We present results from groundbased optical and IUE UV observations of five bright blue stars in M33 and four in M31. Four of the M33 stars are in two close pairs. We discuss these together with the previously published results on five stars. We discuss the limits of feature detectability in faint IUE spectra. Where possible, we have derived spectral classifications and fit continuum models to the data, from U-V and visible range spectra, and visible range photometry. The stars are all O, B or W-R type of high luminosity. As reported previously, we have continued to find that, compared with Galactic counterparts, the UV line spectrum and the stellar wind indicators are weak. We have also found that the UV extinction law in both M33 and M31 stars is steeper than the Galactic law, and similar to that found in the LMC. We discuss the evidence for stellar and extinction anomalies in our Galaxy, and conclude that our sample stars are disk objects in M33 and M31.