

Paper: 
The Upward Diffusion of Hydrogen in HeliumRich Subdwarf B Stars 
Volume: 
334, 14th European Workshop on White Dwarfs 
Page: 
297 
Authors: 
Unglaub, K. 
Abstract: 
For the example T_{eff} = 35000 K, log g = 6.0 the effect of diffusion and weak winds on the chemical composition of helium and carbonenriched subdwarf B stars is investigated with an initial composition as predicted by Cassisi et al. (2003) for the helium flash induced mixing in Population II stars. The elements H, He, C, N and O are taken into account simultaneously in the calculations for time scales of 10^{8} yr. From theoretical considerations mass loss rates in the range 10^{−14}M_{Solar}/yr ≤ dM/dt ≤ 10^{−12}M_{Solar}/yr are plausible for the given stellar parameters. With an initial number ratio H/He = 1.7 × 10^{−3} the upward diffusion of hydrogen leads to an increase of the H/He ratio between a factor of ten for dM/dt = 10^{−12}M_{Solar}/yr and a factor of about 200 to H/He ≈ 0.4 for dM/dt = 10^{−14}M_{Solar}/yr. For an initial ratio of H/He larger by a factor of ten, the upward diffusion of hydrogen could explain the H/He ratios in "blue hook" stars of globular clusters analyzed by Moehler et al. (2004). 
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