Physical parameter eclipse mapping of the quiescent disc in V2051 Ophiuchi

Abstract

We analyse simultaneous UBVR quiescent light curves of the cataclysmic variable V2051 Oph using the Physical Parameter Eclipse Mapping (PPEM) method in order to map the gas temperature and surface density of the disc for the first time. The disc appears optically thick in the central regions, and gradually becomes optically thin towards the disc edge or shows a more and more dominating temperature inversion in the disc chromosphere. The gas temperatures in the disc range from about 13 500 K near the white dwarf to about 6000 K at the disc edge. The intermediate part of the disc has temperatures of 9000 to 6500 K. The quiescent disc (chromosphere) shows a prominent bright region with temperatures of 10 500 K around the impact region of the stream from the secondary with an extension towards smaller azimuths. The disc has a size of 0.53±0.03Rℒ1 and a mass accretion rate ℳ˙=1015 to 1017 g s−1. The light curves must include an uneclipsed component, a hot chromosphere and/or a disc wind. The PPEM method allows us to determine a new distance of 146±20 pc, compatible with previous rough estimates. For the white dwarf we then reconstruct a temperature of 19 600 K, if the lower hemisphere of the white dwarf is occulted by the disc. We suggest that the accretion disc is a sandwich of a cool, optically thick central disc with hot chromospheric layers on both sides, as was suggested for HT Cas. This chromosphere is the origin of the emission lines. We find that although V2051 Oph is very similar to the SU UMa-type dwarf novae HT Cas, OY Car and Z Cha, there must be a substantial difference in order to explain its unique light curve. The reason for the difference could be a higher mass transfer rate caused by the more massive secondary, and/or a small but significant magnetic field of the white dwarf, just strong enough to disrupt the innermost disc.