Physical properties of Wolf-Rayet stars

The striking broad emission line spectroscopic appearance of Wolf-Rayet(WR) stars has long defied analysis, owing to the extreme physical conditions within their line- and continuum-forming regions. Recently, model atmosphere studies have advanced sufficiently to enable the determination of stellar temperatures, luminosities, abundances, ionizing fluxes, and wind properties. The observed distributions of nitrogen- (WN) and carbon (WC)-sequence WR stars in the Milky Way and in nearby star-forming galaxies are discussed; these imply lower limits to progenitor masses of similar to 25, 40, and 75 M circle dot for hydrogen-depleted (He-burning) WN, WC, and H-rich (H-burning) WN stars, respectively. WR stars in massive star binaries permit studies of wind-wind interactions and dust formation in WC systems. They also show that WR stars have typical masses of 102 25 M circle dot, extending up to 80 M circle dot for H-rich WN stars. Theoretical and observational evidence that WR winds depend on metallicity is presented, with implications for evolutionary models, ionizing fluxes, and the role of WR stars within the context of core-collapse supernovae and long-duration gamma-ray bursts.