The dwarf nova SS Cygni: what is wrong?

¹ Departamento de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile e-mail: Matthias.Schreiber@uv.cl
² Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université P. et M. Curie, 98bis boulevard Arago, 75014 Paris, France e-mail: lasota@iap.fr
³ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland

Received: 23 June 2007
Accepted: 3 August 2007

Abstract

Context.Since the Fine Guiding Sensor (FGS) on the Hubble Space Telescope (HST) was used to measure the distance to SS Cyg to be  pc, it became apparent that at this distance the disc instability model fails to explain the absolute magnitude during outburst. It remained, however, an open question whether the model or the distance have to be revised. Recent observations led to a revision of the system parameters of SS Cyg and seem to be consistent with a distance of  pc.

Aims.We re-discuss the problem taking into account the new binary and stellar parameters measured for SS Cyg. We confront not only the observations with the predictions of the disc instability model but also compare SS Cyg with other dwarf novae and nova-like systems.

Methods.We assume the disc during outburst to be in a quasi stationary state and use the black-body approximation to estimate the accretion rate during outburst as a function of distance. Using published analysis of the long term light curve we determine the mean mass transfer rate of SS Cyg as a function of distance and compare the result with mass transfer rates derived for other dwarf novae and nova-like systems.

Results.At a distance of  pc, both the accretion rate during outburst as well as the mean mass transfer rate of SS Cyg contradict the disc instability model. More important, at such distances we find the mean mass transfer rate of SS Cyg to be higher or comparable to those derived for nova-like systems.

Conclusions.Our findings show that a distance to SS Cyg 140 pc contradicts the main concepts developed for accretion discs in cataclysmic variables during the last 30 years. Either our current picture of disc accretion in these systems must be revised or the distance to SS Cyg is ~100 pc.