The effect of crystallization on the pulsations of white dwarf stars

We consider the pulsational properties of white dwarf star models with temp eratures appropriate for the ZZ Ceti instability strip and with masses larg e enough that they should be substantially crystallized. Our work is motiva ted by the existence of a potentially crystallized DA variable (DAV), BPM 3 7093, and the expectation that digital surveys in progress will yield many more such massive pulsators. A crystallized core makes possible a new class of oscillations, the torsional modes, although we expect these modes to co uple at most weakly to any motions in the fluid and therefore to remain uno bservable. The p-modes should be affected at the level of a few percent in period, but are unlikely to be present with observable amplitudes in crysta llizing white dwarfs any more than they are in the other ZZ Ceti's. Most re levant to the observed light variations in white dwarfs are the g-modes. We find that the kinetic energy of these modes is effectively excluded from t he crystallized cores of our models. As increasing crystallization pushes t hese modes farther out from the center, the mean period spacing (AP) betwee n radial overtones increases substantially with the crystallized mass fract ion, M-cr/M*. In addition, the degree and structure of mode trapping is aff ected. The fact that some periods are strongly affected by changes in the c rystallized mass fraction while others are not suggests that we may be able to disentangle the effects of crystallization from those due to different surface layer masses.