1998A&A...340..617M

This paper presents a new method for the calculation of LTE model atmospheres and their theoretical spectra, under conditions relevant to hot white dwarfs. The new method rigorously takes into account noncoherent (Compton) scattering of isotropic radiation by free electrons. Scattering is described by the Fokker-Planck expansion of the full scattering kernel, valid at hν≪mc² and kT≪mc² (Kompaneets equation). Iterations of a model atmosphere are performed following the Rybicki numerical scheme, supplemented by noncoherent scattering terms. The constraint of radiative equilibrium has been linearized with respect both to the Planck function and the noncoherent scattering term simultaneously. Such a double linearization yields rapid temperature corrections, particularly in the uppermost scattering dominated layers of a stellar atmosphere. The code has been used to compute sample series of pure H models at T_eff=10⁵K and various logg. The models clearly exhibit the typical effects of Compton scattering in a stellar atmosphere, i.e. the rise of gas temperature in the uppermost, mostly scattering layers, and significant deficiency and flux cutoff in the emerging X-ray spectrum. The paper presents also Comptonized model spectra of two hot hydrogen-rich (DA) white dwarfs: HZ43 and PG0824+289, in which the helium number abundance is exceptionally low, N_He/N_H≤10^–7. Compton scattering has practically no impact on UV and EUV spectra of both stars as seen by IUE, ROSAT WFC, and EUVE detectors. However, in both stars there exists the predicted decrease of X-ray flux below 50Å and the cutoff at 40 and 39Å (0.3keV), respectively, which is in the range of e.g. ROSAT PSPC and HRI instruments. The effect is restricted to pure hydrogen atmospheres of both stars and vanishes when N_He/N_H>10^–3. Compton scattering of X-rays is therefore equivalent to some additional opacity in that spectral region, which appears only in DA white dwarfs. On the other hand, Compton scattering cannot account for the ``missing'' opacity in EUV and UV spectra of these stars.