A Mechanism for the Outbursts of U Geminorum Stars
Abstract
Adopting Kraft's close-binary model for dwarf novae, we propose a model for the steady-state structure of the envelope of the cool component, which we assume to be steadily losing mass to its companion. We suppose that the shear flow over the surface due to this mass loss is turbulent, and we therefore include shear-flow turbulence as a mechanism of energy transport in the convection zone. The stability of such models to finite perturbations in the mixing length for shear-flow turbulence (equivalent to perturbations in the mean horizontal flow) is explored. Some models relax to the original steady-state solutions on the Kelvin time scale of the surface convection zone; other models undergo sustained thermal-relaxation oscillations. For unstable models maximum light occurs when the cool component overflows the Roche limit by a large amount, for then the energy transport is greatly enhanced by shear-flow turbulence; minimum light, on the other hand, occurs when the star's radius falls below the Roche limit, for then there is no transport by shear-flow turbulence. The light curves have cycles of about 15 days and amplitudes of 1.5 mag, in qualitative agreement with some of the small-amplitude U Gem stars.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 1970
- DOI:
- 10.1086/150694
- Bibcode:
- 1970ApJ...162..621O