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

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5. Conclusions

Our computations show that mass flow via a vertically extended hot corona above the cool disk has three consequences as following.

5.1. UV-delay

A self-sustained corona disk can lead to an evaporation of the inner disk region. This is the part from which the UV is emitted during the outburst, the state of high mass flow in the disk. If a hole has been formed during quiescence it has first to be filled again in the next outburst before this radiation can appear. This needs a mass diffusion time and results in a lag of the rise in UV light compared to the optical which comes from more outer disk regions. This provides a natural explanation for the delay between optical and UV radiation at rise to outburst observed for some dwarf novae.

5.2. X-rays during quiescence

The evaporation rate changes with the size of the hole and thus does the accretion onto the white dwarf fed by this process. In the corona and at the white dwarf X-rays are produced, the major part coming from the white dwarf boundary layer (Liu et al. 1995). We expect a decreasing X-ray emission during the creation of the hole. Further changes depend on the situation in the cool disk, whether the inner disk boundary stays at a fixed location. These changes explain the variation of X-ray emission observed for many dwarf novae as we have shown in la Dous et al. (1997). We point out that the creation of a hole is an event in the early quiescence. Whether a hole is created at all depends on the mass distribution left over from the last outburst. ROSAT observations for the dwarf nova Z Cha taken weeks to months after the last outburst therefore can not show signatures of hole formation. Van Teeseling (1997) argued that a decreasing flux should be expected due to the evaporation model. But this is not the case so late after the outbursts. The observations are in agreement with the theory for a phase of accretion via the corona at a moderate rate during late quiescence.

5.3. Onset of the outburst

The accumulation of the matter in the cool disk is changed. Without evaporation the recent computations for dwarf nova outbursts (Cannizzo 1993, Ludwig et al. 1994, Ludwig & Meyer. 1997) all show a tendency that the onset of instability occurs near the inner edge of the disk (which is close to the white dwarf in these investigations). In our computations, including evaporation, this is different, for the chosen parameters the onset of the outburst occurs at [FORMULA]. The effective temperature is 6023 K where the disk starts to turn into the hot state. This onset of the instability away from the inner edge of the disk is in better agreement with the observation.

Evaporation of an inner accretion disk region might also be expected in other close binary systems where the disk is cool, as in black hole transient sources.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998