Context. Radiation from accretion discs in cataclysmic variable stars (CVs) provides fundamental information about the properties of these close binary systems and about the physics of accretion in general. Of particular interest are dwarf-nova outburst cycles during which variations of the disc properties allow a detailed study of the physical processes in accretion flows.
Aims: The detailed diagnostics of accretion disc structure can be achieved by including in its description all the relevant heating and cooling physical mechanism, in particular the convective energy transport that, although dominant at temperatures ≲10⁴ K, is usually not taken into account when calculating spectra of accretion discs. The disc's self-consistently calculated structure and emission allow testing models of dwarf-nova outbursts and accretion-disc models in general.
Methods: We constructed a radiative transfer code coupled with a code determining the disc's hydrostatic vertical structure.
Results: We have obtained for the first time a model spectra of cold, convective accretion discs. As expected, these spectra are mostly flat in the optical wavelengths with no contribution from the UV, which in quiescence must be emitted by the white dwarf. The disc structures obtained with our radiative-transfer code compare well with the solutions of equations used to describe the dwarf-nova outburst cycle according to the thermal-viscous disc instability model thus allowing the two to be combined. For high-temperature radiative discs our spectra are compatible with models obtained with Hubeny's code TLUSTY.
Conclusions: Our code allows calculating the spectral evolution of dwarf nova stars through their whole outburst cycle, providing a new tool for testing models of accretion discs in cataclysmic variables. We show that convection plays an important role in determining the vertical disc structure and substantially affects emitted spectra when, as often the case, it is effective at optical depths τ ~ 1. The emergent spectrum is independent of the parameters of the convection model. We confirm that, as required by the disc instability model, quiescent discs in dwarf novae must be optically thick in their outer regions. In general, no emission lines are present in the absence of external irradiation.