EVOLUTION OF HELIUM WHITE-DWARFS WITH HYDROGEN ENVELOPES

The present study is aimed at exploring the effects of hydrogen envelo pes on the structure and evolution of low-and intermediate-mass, heliu m white dwarfs. To this end, we compute the evolution of models of hel ium white dwarfs with masses ranging from 0.15 to 0.5 M. for low and i ntermediate effective temperatures. We treat the mass of the hydrogen envelope as a free parameter within the range 10(-8)less than or equal to M-H/M less than or equal to 4 x 10(-3). The calculations are carri ed out by means of a detailed white dwarf evolutionary code in which u pdated radiative opacities and equations of state for hydrogen and hel ium plasmas are considered. The energy transport by convection is desc ribed by the full spectrum turbulence theory developed by Canuto, Gold man & Mazzitelli, which has no free parameters. We also take into acco unt both convective mixing in the outer layers occurring at low lumino sities and the presence of hydrogen burning at the bottom of the hydro gen-rich envelope by means of the inclusion of a detailed network of t hermonuclear reaction rates. Our attention is focused mainly on that p hase of evolution where finite-temperature effects are particularly si gnificant. In this respect, we find that thick hydrogen envelopes appr eciably modify the radii and surface gravity of the no-hydrogen models , especially in the case of low-mass configurations. In addition, conv ective mixing in low-luminosity models with thin hydrogen envelopes le ads to objects with helium-dominated outer layers. Finally, we find th at the role played by hydrogen burning in these stars is strongly depe ndent on the mass of the hydrogen envelope. The computations presented here represent the most detailed models of helium white dwarfs with h ydrogen envelopes presently available. These models should be particul arly valuable for the study of the structure and evolutionary status o f the recently detected low-mass white dwarfs in binary systems.