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Paper: The Mass Loss Rates of sdB Stars
Volume: 392, Hot Subdwarf Stars and Related Objects
Page: 95
Authors: Unglaub, K.
Abstract: According to previous investigations the effect of diffusion in the stellar atmospheres and envelopes of subdwarf B (sdB) stars with luminosities 10 ≤ L/L ≤ 100 strongly depends on the presence of weak winds with mass loss rates dM/dt ≤ 10−12M/yr. These calculations with the mass loss rate as a free parameter have shown that it is hardly possible to reproduce the measured abundances of helium and metals simultaneously. A possible reason is the decoupling of metals, which preferably absorb the photon momentum, from hydrogen and helium in the wind region. In the present paper it will be investigated if “chemically homogeneous” winds, as assumed in previous investigations, with mass loss rates dM/dt ≤ 10−12M/yr can exist. From an observational point of view the existence of weak winds in sdB stars is unclear. Only in the most luminous ones possible wind signatures have been detected. Therefore it will be investigated if according to the theory of radiatively driven winds the existence of weak winds is plausible. A stellar mass M = 0.5M is assumed.

The results for effective temperatures Teff = 35000, 30000 and 25000K, metallicities 0.1 ≤ Z/Z ≤ 1 predict decreasing mass loss rates with increasing surface gravity. Dependent on the luminosity and metallicity the mass loss rates are between about 10−11M/yr and zero. If at all, chemically homogeneous winds can exist for the most luminous sdB stars only. For the other ones selective winds are expected which should lead to additional changes of the surface composition.

In sdB stars, hot white dwarfs and HgMn stars (which are chemically peculiar main sequence stars) the measured metal abundances are tendencially lower than the ones predicted from diffusion calculations which assume an equilibrium between gravitational settling and radiative levitation. Only for helium in almost all cases the measured abundances are larger than the predicted ones, but usually lower, below the solar value. This may be an indication that the abundance anomalies of metals are preferably due to the selective winds, whereas the helium deficiencies are due to gravitational settling, which for still unknown reasons is less effective than expected in an undisturbed stellar atmosphere.

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