LTE model atmospheres for accreting white dwarfs

• Published in: Monthly notices of the Royal Astronomical Society Vol. 226; no. 3; pp. 725 - 738
• Main Authors: Williams, G. A., King, A. R., Brooker, J. R. E.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford University Press 01-06-1987; Blackwell Science
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Faint blue stars (including blue stragglers), white dwarfs, degenerate stars, nuclei of planetary nebulae; Normal stars (by class): general or individual; Stars; Stellar atmosphere; White dwarf; Local thermodynamic equilibrium; Atmosphere model
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/226.3.725

We present the results of self-consistent atmosphere calculations appropriate for the optically thick regions of accretion flows on to white dwarfs, and which can be used to model the observed soft X-ray – EUV spectra of cataclysmic variables. The calculations take into account irradiation by hard X-rays from other parts of the accretion flow, and the pressure of the accretion flow itself. We show that radiation pressure limits the effective temperature Teff of stable atmospheres to $kT_\text{eff} \lesssim 20-40 \,\text{eV},$ depending on the white dwarf mass: the approximation that the atmosphere is fully ionized and that the Eddington limit applies is shown to be incorrect. We evaluate blackbody fits to the soft X-ray continua of the atmospheres. These consistently overestimate Teff, but underestimate the emitting area, in such a way that the total luminosity of the soft X-ray component is usually misestimated only by factors ~2. We comment on the implications for the soft X-ray excess of magnetic cataclysmic variables.