Hot white dwarfs and the UV delay in dwarf novae

Abstract

We calculate the effect of illumination of dwarf nova accretion discs by radiation from a hot, central, white dwarf. We show that, only for very hot white dwarfs (Teff ≈ 40 000 K), the inner regions of quiescent dwarf nova discs are partially depleted so that the delay between the rise to outburst of the optical and UV fluxes would be increased as suggested recently by King. This depletion, however, must create several small outbursts between the main outbursts, contrary to observations. Lower white dwarf temperatures may cause the outbursts to be of the 'inside-out' type, thus removing the UV delay. We conclude that white dwarf irradiation of dwarf nova discs is not very efficient because, for example, the UV radiation from the hot white dwarf does not penetrate deep enough in the disc atmosphere. The total ablation of the inner disc by, e.g., evaporation (possibly related to illumination) appears to be a very promising possibility, accounting for both the extreme UV (EUV) delay and the general properties of the light curves.