Implications of the HST/FGS parallax of SS Cygni on the disc instability model

Abstract

We analyse the consequences of the recently measured parallax of SS Cygni (Harrison et al. 1999) on the accretion disc limit cycle model. Using the observed long term light curve of SS Cyg and d = 166±12 pc, we obtain for the mean mass transfer rate Ṁ̄tr = 4.2±1.7 × 1017 gs-1. In addition, we calculate the vertical structure of the accretion disc taking into account heating of the outer disc by the stream impact. Comparing the mean accretion rate derived from the observations with the calculated critical mass transfer rate, we find that the disc instability model disagrees with the observed long term light curve of SS Cyg as Ṁ̄tr is greater or similar to the critical mass transfer rate. The failure of the model indicated by this result can be confirmed by considering that the accretion rate at the onset of the decline should bc exactly equal to the value critical for stability. In contrast to this prediction of the model, we find that the accretion rate required to explain the observed visual magnitude at the onset of the decline must be significantly higher than the critical mass transfer rate. Our results strongly suggest that either the usually assumed temperature dependence of the viscosity parameter α is not a realistic description of the disc viscosity, that the mass transfer rate in SS Cyg noticeably increases during the outbursts or, finally, that the HST distance of 166±12 pc, is too high.