Spectral evolution of accretion disks of dwarf novae. III - Outburst cycle of SS Cygni

Abstract

A modified disk atmosphere has been used to produce absorption profiles for H-beta, H-gamma, and He I 4471 A lines to fit those at the maximum light during the October 1981 outbursts of SS Cyg and those during the decline. The coexistence of strong emission cores and absorption wings at the maximum light shows that the mass transfer rate at SS Cyg in the outburst was about 5 x 10 exp 17 g/s. The viscosity parameter in the disk during quiescence is between 0.006 and 0.6. The weak emission and shallow absorption during rise indicate that the outburst is initiated from the inner region of the accretion disk during the outburst. This result is compatible with the disk thermal instability hypothesis for CV outbursts, but incompatible with the mass transfer instability scenario for CV outbursts.