LTE model atmospheres for hot, helium-rich degenerate stars

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Abstract
A new grid of hot, high-gravity LTE model atmospheres has been developed for the analysis of EXOSAT soft X-ray observations of He-rich degenerates. H, He, C, N and O opacity sources are included in the calculation and their distribution within the model stellar envelopes is homogeneous. Since He-rich degenerates have an extremely uncertain composition, these models span a uniquely large range of compositions in comparison with earlier publications – CNO:He varying from 0.01 to 80 times the solar value (by number) and H:He from 0 to 10 times solar abundance. The grid was calculated for temperatures of 50 000–200 000 K. A value of log g = 7 was used in most cases but it ranged from 5–8 for selected compositions. The emergent fluxes are presented for all the models calculated and the effect of surface gravity and composition on the emergent fluxes is discussed. A number of the models are compared with other published calculations to verify the code used and to assess the importance of non-LTE effects upon the emergent soft X-ray fluxes at high temperature and gravity.

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