Tidal Synchronization, Heating, and Novae in Binary White Dwarfs
January 21, 2013
Abstract: In compact white dwarf (WD) binary systems (with periods ranging from minutes to hours), dynamical tides involving the excitation and dissipation of gravity waves play a dominant role in determining the physical conditions (such as rotation rate and temperature) of the WDs prior to mass transfer or binary merger. We calculate the amplitude of the tidally excited gravity waves as a function of the tidal forcing frequency for realistic WD models, under the assumption that the outgoing propagating waves are efficiently dissipated in outer layer of the star by nonlinear wave breaking. In our WD models, gravity waves are launched at composition gradients and propagate outwards. We study the effects of dynamical tides on the long-term evolution of WD binaries prior to mass transfer or merger. Above a critical orbital frequency, corresponding to an orbital period of about one hour (depending on WD models), dynamical tides efficiently drive the WD towards synchronous rotation, although a small, almost constant degree of asynchronization is maintained even at the smallest binary periods. We also investigate the effect of tidal heating on the WD (e.g., its internal structure, surface temperature, luminosity, etc.) by including tidal heating in stellar evolution calculations. We compare our predictions to observations of compact WD systems such as the 12.75 minute system SDSS J0651+2844, and we discuss the implications. Finally, we examine the possibility that tidal heating may trigger run-away hydrogen shell burning on the surface of a WD, leading to a nova-like event before the WD merging event.