Time-Dependent Studies of Dynamical Instabilities in Semidetached Binary Systems

Both linear and nonlinear time<1ependent methods are used to investigate the stability of contact members of semidetached binary stars to dynamical perturbations. Assuming a heuristic surface boundary condition of constant pressure at a fixed radius and adiabatic spherically symmetric motion, the linear analysis shows stellar envelopes in such systems to be unstable in a significant region of the HertzsprungRussell diagram. With the assumption of ionization equilibrium during the outburst, a nonlinear hydrodynamic, but adiabatic, analysis shows that the instability results in the rapid shedding of material from the surface and the setting up of adiabatic gradients within the star. Estimated variations in surface luminosity are similar to the changes observed in U Geminorum stars, but not novae, and the reasons for this are discussed.