Evolution of helium white dwarfs with hydrogen envelopes

Abstract

The present study is aimed at exploring the effects of hydrogen envelopes on the structure and evolution of low- and intermediate-mass, helium white dwarfs. To this end, we compute the evolution of models of helium white dwarfs with masses ranging from 0.15 to 0.5 M⊙ for low and intermediate effective temperatures. We treat the mass of the hydrogen envelope as a free parameter within the range 10−8≤MH/M≤4×10−3. The calculations are carried out by means of a detailed white dwarf evolutionary code in which updated radiative opacities and equations of state for hydrogen and helium plasmas are considered. The energy transport by convection is described by the full spectrum turbulence theory developed by Canuto, Goldman & Mazzitelli, which has no free parameters. We also take into account both convective mixing in the outer layers occurring at low luminosities and the presence of hydrogen burning at the bottom of the hydrogen-rich envelope by means of the inclusion of a detailed network of thermonuclear reaction rates. Our attention is focused mainly on that phase of evolution where finite-temperature effects are particularly significant. In this respect, we find that thick hydrogen envelopes appreciably modify the radii and surface gravity of the no-hydrogen models, especially in the case of low-mass configurations. In addition, convective mixing in low-luminosity models with thin hydrogen envelopes leads to objects with helium-dominated outer layers. Finally, we find that the role played by hydrogen burning in these stars is strongly dependent on the mass of the hydrogen envelope. The computations presented here represent the most detailed models of helium white dwarfs with hydrogen envelopes presently available. These models should be particularly valuable for the study of the structure and evolutionary status of the recently detected low-mass white dwarfs in binary systems.