Mon. Not. R. Astron. Soc., 404, 1698-1710 (2010/June-1)
Stellar winds and mass loss from extreme helium stars.
JEFFERY C.S. and HAMANN W.-R.
Abstract (from CDS):
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 of 10–10-10–7 M☉yr–1. These rates follow a Castor-type relation marking a lower limit for the mass loss from hot stars of all kinds. The mass-loss rates of the stars studied 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.