Stellar Populations in M33 and M31

摘要

The UV-brightest stars in M31 and M33, partly chosen from UV images of M31 and M33 obtained with the Ultraviolet Imaging Telescope, were observed 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 favorable candidates for HST UV spectroscopy to study the stellar winds of hot stars in galaxies of the Local Group; With HST, we obtained high resolution spectra with the Faint Object Spectrograph (FOS) of the most luminous hot stars (O-B supergiants, WR, LBV) in M31 and M33, in the range 1200-3300A. From HST UV spectra and complementary optical data (KPNO, CFHT, IAC-WHT) we derived the stellar parameters: Teff, Lbol, g, R, abundances, mass loss rate and wind velocity. We analyzed the UV wind lines with the SEI method to derive wind velocity. From observations of the H-alpha emission line, and the H-gamma profile at high resolution, which are not affected by ionization uncertainties as the W ions, we very accurately derived the mass loss rate and the gravity. We obtained these spectra at the 4.2m WHT tel. at IAC (Canary Island, Spain), and we analyzed them with a SEI-Balmer method (Bianchi et al.1994) and non-LTE model atmospheres. Spectra and derived parameters of M31 and M33 stars were compared with Galactic and LMC stars of the same type, and with theoretical predictions. We also derived properties of the intervening ISM, both in the host galaxy and in the Milky Way halo.