Stellar Winds and Mass Loss from Extreme Helium Stars
C.S. Jeffery1 and W.-R. Hamann2
2 Institut für Physik und Astronomie, Universität Potsdam, Germany
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 Msun yr-1. These rates follow a Castor-type (dM/dt ~ L1.5) 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.
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