Model Spectra for the Helium Cataclysmic Variable: AM CVn

Abstract

Accretion through Roche lobe overflow in the helium cataclysmic binary AM CVn is considered and its emitted spectrum is computed using model stellar atmospheres. The accretion disc in AM CVn is helium-rich and such discs have hitherto not been constructed. We have computed the models by numerically solving the radiative transfer equation for helium opacities. The method adopted is that of treating the accretion disc as a set of optically thick and geometrically thin concentric annuli. Each annulus is treated individually as a blackbody or as a stellar atmosphere. We find that our description of AM CVn is consistent with both photometric and spectroscopic measurements of the system which have been made using the Nordic Optical Telescope (NOT) and the International Ultraviolet Explorer (IUE). We find that the stellar model synthetic spectrum provides a better fit to the observed spectrum than the Planckian disc spectrum. Also, for a given accretion rate, the synthetic spectrum composed of the sum of a series of stellar atmospheres is flatter than the corresponding blackbody sum.