HOT OR NOT - THEORETICAL BLUE EDGES FOR DA AND DB WHITE-DWARF MODELS

We present the nonadiabatic results of a parametric survey of compositionally stratified evolutionary white dwarf models of with helium surface layers (DB white dwarfs) and hydrogen surface layers (DA white dwarfs). We examine the effect of varying the stellar mass, varying the hydrogen and helium layer masses, and the treatment of convection on the temperature of the theoretical blue edge. In general, the blue edge is relatively insensitive to the helium or hydrogen layer mass. The exception occurs for DB models with helium layer masses less than 10(-8)M*; they are pulsationally stable at all temperatures. We find the blue edge is very sensitive to the treatment of convection, consistent with previous results. The most efficient treatment of convection we consider yields a blue edge near 27,000 K for our 0.6 M. DB model and near 12,700 K for our 0.6 M. DA models. Thus, we can match the observed DB blue edge located between 24,000-25,000 K by tuning the convective efficiency. The temperature of our theoretical DA blue edges for 0.6 M. models with efficient version of convection are similar to the observed blue edge. For both DA and DB models, there is a slight dependence of the blue edge on stellar mass, with higher mass models having hotter blue edges. The change in blue edge with stellar mass may offer an explanation for the existence of nonpulsators within the instability strip.