Theory and observations of the optical continuum and line spectra of accretion discs around white dwarfs

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Abstract
Theoretical calculations of the optical continuum (λ ≳ 3000 Å), UBV colours and Hα, Hβ and Hγ absorption line profiles are presented for steady-state accretion discs around white dwarfs. Several approximations (in particular with regard to the atmospheres) are made and discussed. The uncertainties with regard to disc Z-structure and outer boundary are considered. The effects of varying the inclination angle, mass of the white dwarf and mass flux rate through the disc are discussed. The theoretical results are compared with observations of UBV colours obtained from the literature and with line profiles reported here of dwarf-novae in outburst and UX UMa ‘disc’ stars. In general the overall agreement between theory and observations is good for the UBV colours and reasonable for the line profiles. Comparison between the outburst and the superoutburst properties of dwarf novae suggests that the mass flux rate is of order four times larger during the superoutbursts. Comparison is also made with the theoretical disc spectra results of Schwarzenberg-Czerny & Rózyczka and Herter et al. Reasonable agreement is found and reasons are suggested for the overall differences between some of those models and the ones reported here.

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