Wind-dominated optical line emission from accretion disks around luminous cataclysmic variable stars

Abstract

CATACLYSMIC variable stars are thought to consist of a low-mass star orbiting a more massive white-dwarf primary star. Gas from the secondary star flows onto the white dwarf by way of an accretion disk, and the recurrent outbursts that characterize these objects are attributed to either instabilities in the accretion disk (in the case of dwarf novae) or thermonuclear explosions of material accumulated on the white dwarf (for novae). Observations^1,2 of low-luminosity cataclysmic variables have revealed double-peaked emission lines consistent with the accretion-disk picture. But the emission lines associated with high-luminosity cataclysmic variables are single-peaked^3–9, which poses a considerable challenge for the disk theory^10,11. It has been suggested that winds from the accretion disk, driven by radiation pressure, might be responsible for altering the emission-line characteristics^4,10. Here we present a model of the disk wind in which the emission lines arise in a thin layer where the wind emerges from the disk. In our model, the accelerating wind introduces a pronounced anisotropy in the optical depth of the disk, from which single-peaked emission lines naturally arise.