EFFECTS OF COMPTON-SCATTERING IN ATMOSPHERES OF HTO WHITE-DWARFS

Model atmospheres of white dwarf stars are routinely computed taking i nto account coherent (Thomson) scattering on free electrons. This pape r presents a set of LTE model atmospheres computed with the account of noncoherent Compton scattering, which in a physically correct way des cribes photon frequency changes at the time of scattering. Effective t emperatures T(eff) of white dwarfs chosen here are in the range 1 x 10 (5) K to 2 x 10(5) K, and are relevant to the hottest known white dwar f stars. Surface gravity ranges from log g = 8.0 down to 5.2 (c.g.s. u nits). Model atmospheres assume pure hydrogen, mixed hydrogen/helium a nd pure helium chemical composition, thus corresponding to DA, DAO, an d DO classes of white dwarfs. Most of the model atmospheres exhibit di stinct effects of Compton scattering, which cause a huge rise of tempe rature T in surface layers, as compared with models assuming Thomson s cattering of continuum photons. Simultaneously, Compton scattering imp lies a depression of the emerging fluxes in X-ray domain (lambda less- than-or-equal-to 50 angstrom), since high energy photons are efficient ly redistributed to lower energies after scattering with cooler electr on gas. Both the above effects are strongest in pure H models of low g ravity, approaching the critical gravity g(cr). Mixed He/H and pure He models exhibit weaker effects of Compton heating and flux depression in X-rays. High gravity models of log g = 8.0 are very weakly altered by Comptonization in models of any T(eff) and chemical composition. Th eoretical X-ray spectra which are presented here yet cannot account fo r the observed deficiency of hot DA white dwarfs in the ROSAT X-ray an d EUV surveys. However, significant heating of some model atmospheres in the uppermost layers could have impact on the profiles of the stron gest spectral lines.