EVOLUTION OF DA WHITE-DWARFS IN THE CONTEXT OF A NEW THEORY OF CONVECTION

In this study we compute the structure and evolution of carbon-oxygen DA (hydrogen-rich envelope) white dwarf models by means of a detailed and updated evolutionary code. We consider models with masses from 0.5 to 1.0 M. and we vary the hydrogen layer mass in the interval 10(-13) less than or equal to M-H/M less than or equal to 10(-4). In particula r, we treat the energy transport by convection within the formalism of the full-spectrum turbulence theory, as given by the Canuto, Goldman & Mazzitelli (CGM) model. We explore the effect of various hydrogen la yer masses on both the surface gravity and the hydrogen burning. Conve ctive mixing at low luminosities is also considered. One of our main i nterests in this work has been to study the evolution of ZZ Ceti model s, with the aim of comparing the CGM and mixing-length theory (MLT) pr edictions. In this connection, we find that the temperature profile gi ven by the CGM model is markedly different from that of the ML1 and ML 2 versions of the MLT. In addition, the evolving outer convection zone behaves differently in both theories. We have also computed approxima te effective temperatures for the theoretical blue edge of the DA inst ability strip by using thermal time-scale arguments for our evolving D A models. In this context, we found that the CGM theory leads to blue edges that are cooler than the observed ones. However, because the det ermination of atmospheric parameters of ZZ Ceti stars is dependent on the assumed convection description in model atmosphere calculations, o bserved blue edges based on model atmospheres computed considering the CGM theory are required in order to perform a self-consistent compari son of our results with observations. Finally, detailed non-adiabatic pulsational computations of ZZ Ceti models considering the CGM convect ion would be necessary to place the results found in this paper on a f irmer basis.