The mode trapping properties of full DA white dwarf evolutionary models

An important aspect of pulsating DA WDs is related to the mode trapping phenomenon, which has been invoked in the past to explain the fact that all the modes predicted by theoretical models are not actually observed in the ZZ Ceti stars (Brassard et al. 1992). In this context, trapped modes (which require low kinetic energy to oscillate) characterized by periods close to the thermal time-scale of the driving region will reach high enough amplitudes for them to be observed. However, evidence exists that casts some doubts on the correlation between observed amplitudes and mode trapping (Bradley 1998). The shape of the chemical interfaces plays an important role in the trapping properties. The existing calculations invoke diffusive equilibrium in the trace element (DETE) approximation to assess the shape of the H/He transition (Brassard et al. 1992). However, equilibrium conditions may not be achieved at the base of massive hydrogen envelopes, even at the characteristic ages of ZZ Ceti stars (see Iben & MacDonald 1985). In view of these concerns, we have recently performed new evolutionary calculations for DA WD stars which take fully into account time dependent element diffusion, nuclear burning and the history of the WD progenitor in a self-consistent way (for details, see Althaus et al. 2002). The present work is aimed at specifically exploring the mode trapping properties of this new generation of models.