OB stars and Luminous Blue Variables

The most direct method of deriving empirical mass-loss rates for hot stars is through analysis of the UV resonance transitions from dominant metal ions. However, difficulties in deriving the wind ionization balance using currently available (trace) ions means that mass-loss rates remain uncertain. With FUSE, additional wind lines are being observed, spanning a much wider range of species. From these data, the degree of ionization can be determined accurately, so that physical and wind properties can be measured with confidence. As part of the VLT FLAMES Massive Star Large Program consortium, notably Rohied Mokiem, I have studied massive stars in the Galaxy and Magellanic Clouds - spanning a factor of 5 in metallicity - so that the variation of mass-loss properties with metal content can be measured, of importance in the study of high redshift galaxies. SMC O stars indeed possess weaker winds than those at Solar composition, with higher temperature for individual O subtypes.

With Danny Lennon and Nolan Walborn I have studied the physical properties of Galactic B stars, revealing good agreement with predicted wind strengths for B0-B0.5 Ia supergiants, after allowance for wind clumping, but winds from B0.7-B3 Ia supergiants are overestimated. The bistability jump in B supergiants - signalling a sudden division in wind to escape velocity - was re-investigated but not confirmed. With Laurent Drissen and Veronique Petit I am monitoring an Luminous Blue Variable star in the Magellanic galaxy NGC 2366 10,000,000 light years away. This star has a B supergiant spectral type except that it is currently undergoing a giant eruption, such that one solar earth mass of material is being ejected every day! Such high and constant mass-loss is exceptional even amongst massive stars, although all are irregular and structured at a basic level. It is believed that such intense episodes of mass-loss are responsible for the removal of hydrogen envelopes in massive stars, causing evolution to the Wolf-Rayet phase.