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We follow the changes in the structure of the accretion disk of the dwarfnova V2051 Oph along two separate outbursts in order to investigate the causesof its recurrent outbursts. We apply eclipse mapping techniques to a set oflight curves covering a normal (July 2000) and a low-amplitude (August 2002)outburst to derive maps of the disk surface brightness distribution atdifferent phases along the outburst cycles. The sequence of eclipse maps of the2000 July outburst reveal that the disk shrinks at outburst onset while anuneclipsed component of 13 per cent of the total light develops. The derivedradial intensity distributions suggest the presence of an outward-movingheating wave during rise and of an inward-moving cooling wave during decline.The inferred speed of the outward-moving heating wave is ~ 1.6 km/s, while thespeed of the cooling wave is a fraction of that. A comparison of the measuredcooling wave velocity on consecutive nights indicates that the cooling waveaccelerates as it travels towards disk center, in contradiction with theprediction of the disk instability model. From the inferred speed of theheating wave we derive a viscosity parameter alpha_{hot} ~ 0.13, comparable tothe measured viscosity parameter in quiescence. The 2002 August outburst hadlower amplitude (\Delta B ~ 0.8 mag) and the disk at outburst maximum wassmaller than on 2000 July. For an assumed distance of 92 pc, we find that alongboth outbursts the disk brightness temperatures remain below the minimumexpected according to the disk instability model. The results suggest that theoutbursts of V2051 Oph are caused by bursts of increased mass transfer from themass-donor star.

A study of the evolution of the accretion disk of V2051 Oph through two outburst cycles