Thermal instability calculations of the ultraviolet delay in dwarf novae

Abstract

ABSTRA C T We study the viscous evolution of dwarf nova accretion discs irradiated by a constanttemperature white dwarf and the accretion luminosity released at the white dwarf surface. We find that irradiation keeps the inner disc fully ionized out to several white dwarf radii, forcing this region to remain in the hot state even during quiescence. The ionized inner disc is connected to the cool outer disc by an extended transition region which is semistable. Insideout outbursts start at the cool edge of this transition region. The rise of the optical flux coincides with the onset of the heating wave, whereas the UV flux only rises 5‐10 h later when the disc gas is accreted on to the white dwarf. With irradiation included the thermal‐viscous disc instability model is thus closer to explaining the observed UV delay at the onset of a dwarf nova outburst. We find that the optical outburst reaches its peak at the time when the heating wave in the disc is reflected at the outer disc edge as a cooling wave. In contrast, the UV peak is close to the peak in the mass accretion rate on to the white dwarf. Accordingly, the UV flux drops after a delay of ,10 h, essentially independently of the irradiation model.