DB WHITE-DWARF EVOLUTION IN THE FRAME OF THE FULL SPECTRUM TURBULENCETHEORY

We present an analysis of the evolution of carbon-oxygen DB white dwar fs (helium-rich envelope) for a wide range of effective temperatures a nd luminosities, To this end, we employ a full stellar evolution code, in which ave include a new equation of state for helium plasmas recen tly developed by Saumon, Chabrier & Van Horn and new OPAL radiative op acities. The most important feature of our models is that the transpor t of energy by convection is described by the full spectrum turbulence theory, In particular, we have adopted two versions of this theory fo r stellar convection: the Canuto & Mazzitelli theory and the more rece nt. self-consistent theory developed by Canuto, Goldman & Mazzitelli. Both theories, which have no free parameters and account for the whole spectrum of turbulent eddies, represent a great improvement compared to the mixing-length theory approach used thus far in almost all white dwarf studies. Neutrino energy losses as well as crystallization were taken into account. In order to explore the sensitivity of our result s to various input model parameters, we vary the model mass from 0.5 t o 1.0 M. in intervals of 0.1 M., and the helium layer mass in the inte rval of 10(-6) less than or equal to M-He/M less than or equal to 10( -2). The emphasis is put mainly on the behaviour of the evolving outer convection zone. In particular, we analyse the dependence of the loca tion of the theoretical blue edge of the instability strip on the vari ous input parameters. We find that the new ingredients we have Incorpo rated in this study - mostly the new formulations for stellar convecti on - lead to theoretical blue edges in agreement with observations of pulsating DB white dwarfs. In this context, the Canuto, Goldman & Mazz itelli self-consistent theory yields theoretical blue edges somewhat h otter than those given by the Canuto & Mazzitelli theory, which is mor e consistent with a recent determination of the effective temperature of the hot DBV GD 358. Contrary to previous results, we find that, acc ording to the new theories for convection, non-variable DB white dwarf s falling within the instability strip cannot be low-mass configuratio ns. In order to compare with previous computations, we Include in our calculations the most common parametrizations of the mixing-length the ory usually employed in almost all previous white dwarf studies. In th is context, we find that the ML2 parametrization provides a reasonable agreement with the observed blue edge for the DB instability strip. H owever, the profile of the outer convective zone given by the mixing-l ength theory is markedly different from that given by both of the new convective formulations.