Spectroscopy of Massive Stars in NGC 6822 and M33

Abstract

We present blue spectra of massive stars in the Local Group galaxies NGC 6822 and M33. For all the targets, selected from our previous Hubble Space Telescope WFPC2 photometry, no previous spectroscopic studies exist, to our knowledge, except for a bright Wolf-Rayet star in NGC 6822. Photospheric parameters T_eff and gravity are derived by a model fit of the Balmer lines and multiband (UV to optical) photometry combined and are compared with evolutionary models. Inferred masses range from ~7 to ~40 M_solar in the sample. From the spectroscopy, we classify the Wolf-Rayet star in NGC 6822 as WN3(-4), with no hydrogen. We estimate its progenitor mass of ~40+/-10 M_solar. For two stars in the NGC 6822 sample, our data are not sufficient to establish whether they are foreground stars toward NGC 6822 or they belong to the galaxy.