EVOLUTION OF HELIUM WHITE-DWARFS OF LOW AND INTERMEDIATE MASSES

We present detailed calculations of the evolution of low-mass, helium white dwarf models with masses from M = 0.1 to M = 0.5 M. at intervals of 0.05 M. and with a metallicity of Z = 10(-3). For this purpose, we have taken fully into account finite-temperature effects by means of a detailed and updated stellar evolutionary code, in which the convect ive energy transport is described according to the new model for turbu lent convection developed by Canuto & Mazzitelli. Furthermore, our cod e considers the most recent opacity data computed by the Livermore Gro up (OPAL data), and also the new equation of state for helium plasmas developed by Saumon, Chabrier, & Van Horn. Neutrino emission is fully taken into account as well. For models with M less than or equal to 0. 3 M. we started our calculations from fully convective models located at the helium-Hayashi line for each configuration, far away from the w hite dwarf regime. By contrast, the evolutionary sequences correspondi ng to 0.35, 0.4, 0.45, and 0.5 M. were started from initial models res embling white dwarf structures. This was necessary in order to avoid t he onset of helium burning. A consequence of this constraint is the ex istence of a ''forbidden region'' in the HR diagram above log (L/L.) = -0.25 and hotter than log T-eff = 4.45, where helium white dwarfs can exist only for brief intervals. All the models were evolved to log (L /L.) = -5. The evolutionary tracks in the HR diagram have been careful ly analyzed, and we found that the convective efficiency affects the t racks noticeably only in the high-luminosity (pre-white dwarf) regime. We also examined the evolution of central conditions, neutrino lumino sity, radii, surface gravity, and ages. Central densities, radii, and surface gravities asymptotically approach the zero temperature Hamada- Salpeter results, as expected. Neutrino losses are important for the m ore massive helium white dwarf configurations and should be taken into account in detailed evolutionary studies of these objects. Finally, t he structure of the outer convective zone was analyzed in both the fra mework of the mixing length theory (for different convective efficienc ies) and the Canuto & Mazzitelli theory. We found that the profile of the outer convective zone given by the Canuto & Mazzitelli model is ve ry different from that given by any version of the mixing length theor y. This behavior is critical for pulsational instability; however, ste llar parameters such as radius and surface gravity are not significant ly affected in the white dwarf domain. These models should be especial ly suitable for the interpretation of the data about the recently disc overed low-mass white dwarfs in systems containing another white dwarf or a millisecond pulsar.