Stellar Winds and Mass Loss from Extreme Helium Stars

Abstract

Extreme helium stars are very rare low-mass supergiants in a late stage of evolution. They are probably contracting to become white dwarfs following a violent phase of evolution which caused them to become hydrogen-deficient giants, possibly R CrB stars. Using the latest generation of models for spherically expanding stellar atmospheres, we set out to measure mass-loss rates for a representative fraction of these stars. We have used high-resolution ultraviolet and optical spectra, and ultraviolet, optical and near-infrared photometry from a variety of archives. Overall atmospheric parameters have mostly been taken from previous analyses and checked for consistency. Mass-loss rates were measured by fitting the P-Cygni and asymmetric profiles of C, N and Si ultraviolet resonance lines and lie in the range 10^{-10} - 10^{-7} M_{\odot} yr^{-1}. These rates follow a Castor-type (\dot{M} \propto L^{1.5}) relation marking a lower limit for the mass loss from hot stars of all kinds. The mass-loss rates of the studied stars also show a strong correlation with their proximity to the Eddington limit. There is no firm evidence for variability in the stellar wind, although photospheric pulsations have been reported in many cases.