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Astron. Astrophys. 329, 559-570 (1998)

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1. Introduction

Dwarf novae are a subclass of cataclysmic variables, i.e. close binaries consisting of a white dwarf primary and a low-mass main-sequence secondary which fills its critical Roche volume. The secondary loses mass through the inner Lagrangian point which then is accreted via an accretion disk by the primary. The defining characteristic of dwarf novae is that phases of low luminosity (quiescence) alternate with phases of high luminosity (outburst) in a semi-regular way. The outbursts which have amplitudes of 2-6 mag in the visual light occur at intervals of a few weeks or months, the maximum brightness lasting a few days to 20 days. There are also exceptions from this typical outburst behavior. For example, the dwarf nova system U Gem, where outbursts typically last 12 days, in October 1985 underwent an optical outburst of 45 days, duration unprecedented in the [FORMULA] yr that the star has been monitored (Mason et al. 1988).

We have investigated outbursts in dwarf nova accretion disks in the framework of the disk instability model. In this model the outburst phenomenon is explained as a cyclic change of the disk structure between a hot and a cool state. Transition waves, which separate a hot bright region from a cool dark region, propagate through the disk and initiate the light variation.

In this paper we perform high resolution calculations that include several effects left out before. We compare our results with those from previous calculations (Ludwig et al. 1994) which used the localized front approximation (Meyer 1984) and with fine mesh calculations by Mineshige (1987) and Cannizzo (1993). Here one has to note, that the results of most previous calculations seriously suffer from too coarse a resolution (see Cannizzo 1993). In particular, then the amount of lateral heat diffusion in the region of a transition wave is inevitably underestimated (see Mineshige 1987). Finally, our calculations allow us to investigate the influence of deviations of the azimuthal velocity from the Keplerian value on the outburst behavior. For this, in addition to the equation of continuity of mass flow and the energy equation we solve the Navier-Stokes equations for the radial flow and the azimuthal velocity. The deviation from the Kepler velocity is mainly produced by pressure forces. Inside transition fronts a steep gradient of the pressure occurs. Here deviations from the Keplerian flow can have an influence on the structure of the front, which may cause changes in the outburst behavior.

The method of our calculation is given in Sect. 2. Results are shown in Sect. 3 followed by a discussion in Sect. 4. A comparison of our results with those of Mineshige (1987), Cannizzo (1993) and Ludwig et al. (1994) is carried out in Sect. 5, and conclusions are given in Sect. 6.

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© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997