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

We present the nonadiabatic results of a parametric survey of composit ionally stratified evolutionary white dwarf models of with helium surf ace layers (DB white dwarfs) and hydrogen surface layers (DA white dwa rfs). We examine the effect of varying the stellar mass, varying the h ydrogen and helium layer masses, and the treatment of convection on th e temperature of the theoretical blue edge. In general, the blue edge is relatively insensitive to the helium or hydrogen layer mass. The ex ception occurs for DB models with helium layer masses less than 10(-8) M; they are pulsationally stable at all temperatures. We find the blu e edge is very sensitive to the treatment of convection, consistent wi th previous results. The most efficient treatment of convection we con sider yields a blue edge near 27,000 K for our 0.6 M. DB model and nea r 12,700 K for our 0.6 M. DA models. Thus, we can match the observed D B blue edge located between 24,000-25,000 K by tuning the convective e fficiency. The temperature of our theoretical DA blue edges for 0.6 M. models with efficient version of convection are similar to the observ ed 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 hotte r blue edges. The change in blue edge with stellar mass may offer an e xplanation for the existence of nonpulsators within the instability st rip.