Computations of Model Atmospheres with Account of Compton Scattering

Statistics – Computation

Scientific paper

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Stars: Atmospheres, White Dwarfs, Radiative Transfer

Scientific paper

In this paper we formulate new method designed for calculation of hot LTE model stellar atmospheres in hydrostatic and radiative equilibrium, which takes into account Compton scattering on free electrons according to the diffusion approximation. The equation of transfer at each frequency includes both temperature corrections and noncoherent scattering terms. Full set of transfer equations and the closing equation of radiative equilibrium is organized in the form extending the Rybicki solution. The corresponding computer code ensures very rapid convergence of iterated model atmospheres. New computer code has been used to calculate series of pure H models in LTE for Teff=5 times 10^4 K, 7 times 10^4 K, and 1 times 10^5 K. Assumed surface gravities log g in a series are in the range between 7.0 and 6.0 -- 5.0 (cgs units). All test models are obtained with the Eddington approximation in the radiation transfer. This choice of parameters corresponds to the central stars of some planetary nebulae and subdwarf O stars. Model atmospheres clearly exhibit both the typical effects of noncoherent Compton scattering, ie the rise of temperature in the uppermost layers and the depression of monochromatic flux in X-ray part of the spectrum. While both effects are of minor significance at log g=7.0 or at the lowest Teff, they grow substantially at lower surface gravities or at Teff = 1 times 10^5 K. The impact of surface Compton heating on the Balmer line profiles and on the formation of the emission core in H_alpha is also demonstrated here. Computational results demonstrate the importance of Compton scattering of continuum radiation for the formation of strong spectral lines at low log g. Moreover, the rise of boundary T(tau) caused by comptonization should influence the formation of stellar wind in low gravity hot stars.

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