Observations of the Atmospheres and Winds of O-stars, LBVs and Wolf-Rayet Stars

Abstract

This review summarises recent studies of O—stars, Luminous Blue Variables (LBVs) and Wolf-Rayet (WR) stars, emphasising observations and analyses of their atmospheres and stellar winds yielding determinations of their physical and chemical properties. Studies of these stellar groups provide important tests of both stellar wind theory and stellar evolution models incorporating mass-loss effects. Quantitative analyses of O—star spectra reveal enhanced helium abundances in Of and many luminous O—supergiants, together with CNO anomalies in OBN and Ofpe/WN9 stars, indicative of evolved objects. Enhanced helium, and CNO—cycle products are observed in several LBV_s, implying a highly evolved status, whilst for the WR stars there is strong evidence for the exposition of CNO—cycle products in WN stars, and helium—burning products in WC and WO stars. The observed wind properties and mass-loss rates derived for O—stars show, in general terms, good agreement with predictions from the latest radiation—driven wind models, although some discrepancies are apparent. Several LBVs show similar mass-loss rates at maximum and minimum states, contrary to previous expectations, with the mechanism responsible for the variability and outbursts remaining unclear. WR stars exhibit the most extreme levels of mass-loss and stellar wind momenta. Whilst alternative mass-loss mechanisms have been proposed, recent calculations indicate that radiation pressure alone may be sufficient, given the strong ionization stratification present in their winds.