Infrared photometry of the dwarf nova V2051 Ophiuchi: I - The mass donor star and the distance

We report the analysis of time-series of infrared $JHK_s$ photometry of the dwarf nova V2051 Oph in quiescence. We modelled the ellipsoidal variations caused by the distorted mass-donor star to infer its $JHK_s$ fluxes. From its infrared colors we estimate a spectral type of $M(8.0\pm 1.5)$ and an equivalent blackbody temperature of $T_\mathrm{BB}=(2700\pm270)\,K$. We used the Barnes & Evans relation to infer a photometric parallax distance of $d_\mathrm{BE}=(102\pm16)$ pc to the binary. At this short distance, the corresponding accretion disc temperatures in outburst are too low to be explained by the disc-instability model for dwarf nova outbursts, underscoring a previous suggestion that the outbursts of this binary are powered by mass-transfer bursts.