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Astron. Astrophys. 328, 247-252 (1997)

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1. Introduction

Dwarf novae show alternating phases of low and high luminosity. In these binaries mass flows over from a Roche Lobe filling main sequence star via an accretion disk onto the white dwarf. During quiescence mass is accumulated in the disk until the next outburst is triggered and the material starts flowing at a rate of about [FORMULA] /yr towards the compact object, leading to a significant increase of luminosity.

The mass flow rate through the cool disk during quiescence is only [FORMULA] /yr to [FORMULA] /yr in the standard model for dwarf nova outbursts. But it could be shown that a self-sustained hot corona exists above this cool inert disk (Meyer & Meyer-Hofmeister 1994, Liu et al. 1995). This corona supports an accretion mass flow of about [FORMULA] /yr and is fed from the cool underlying disk. This leads to an evaporation of the inner disk during quiescence.

The aim of this work is to study the structural evolution of the accretion disk around a 1 [FORMULA] white dwarf including this effect. We evaluate the decrease of surface density starting with the distribution of mass left over from the preceding outburst. The changes of the surface density are computed according to the mass flow rates determined by Liu et al. (1995).

The depletion of matter and formation of a hole in the disk around the white dwarf has important consequences for the next outburst. The inner evaporated part of the disk has to be filled in on the diffusion timescale before it can display the hot UV temperatures during the early outburst (Meyer & Meyer-Hofmeister 1989). This provides a natural explanation for the delay between optical and UV radiation at rise to outburst as observed for several systems (la Dous 1994, Verbunt 1987).

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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