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We are studying the hot star population in nearby galaxies to investigate the dependence of stellar atmospheres and winds on the global characteristics of their host galaxy, in particular the metallicity. Such dependences are predicted by the radiation pressure wind theory, and so far only tested observationally in the Magellanic Clouds (MCs), that are however very different from our own Galaxy. Comparison with stellar population in M31, a spiral supposed to have metallicity and mass similar to the Milky Way, is particularly important.
In support of this research, we observed the UV-brightest stars in M31 and M33, partly chosen from UV images of M31 and M33 obtained with the \it Ultraviolet Imaging Telescope \rm, with ground-based UBV CCD photometry and optical spectroscopy, to accurately determine spectral types, bolometric magnitudes and extinction. In this way we selected the most favourable candidates for HST UV spectroscopy, allowing us to extend our study of the stellar winds in these galaxies.
With HST, we obtained high resolution UV spectra with the Faint Object Spectrograph (FOS), and analysed the wind lines with the SEI method to derive terminal velocity and other wind parameters. These are the first detailed spectra of individual stars in M31 and M33. They also allow for the first time to determine abundances in these galaxies from stellar lines: only information from HII regions was available before, which may not be representative of the stellar content. Finally, by selecting objects at different galactocentric distances we can probe a possible metallicity gradient.
Optical data and UV spectra provide atmospheric parameters of the stars: T$_{eff}$, L, R, and abundances. With complementary observations of the H${\alpha}$ emission line profile at high resolution, and the H${\gamma}$ profile, we derive very accurately the mass loss rate and the gravity. Stellar quantities are compared with galactic stars of the same type.
