Abstract
We provide a fine and homogeneous grid of evolutionary sequences for He-core white dwarfs with masses 0.15-0.45 Msun, including the mass range for ELM white dwarfs (<0.20Msun). The grid is appropriate for mass and age determination, and to study their pulsational properties. White dwarf sequences have been computed by performing full evolutionary calculations that consider the main energy sources and processes of chemical abundance changes during white dwarf evolution. Initial models for the evolving white dwarfs have been obtained by computing the non-conservative evolution of a binary system consisting of a Msun ZAMS star and a 1.4 Msun neutron star for various initial orbital periods. To derive cooling ages and masses for He-core white dwarf we perform a least square fitting of the M(Teff, g) and Age(Teff, g) relations provided by our sequences by using a scheme that takes into account the time spent by models in different regions of the Teff-g plane. This is useful when multiple solutions for cooling age and mass determinations are possible in the case of CNO-flashing sequences. We also explore the adiabatic pulsational properties of models near the critical mass for the development of CNO flashes (~0.2 Msun). This is motivated by the discovery of pulsating white dwarfs with stellar masses near this threshold value. We obtain reliable and homogeneous mass and cooling age determinations for 58 very low-mass white dwarfs, including 3 pulsating stars. Also, we find substantial differences in the period spacing distributions of g-modes for models with stellar masses ~ 0.2 Msun, which could be used as a seismic tool to distinguish stars that have undergone CNO flashes in their early cooling phase from those that have not. Finally, for an easy application of our results, we provide a reduced grid of values useful to obtain masses and ages of He-core white dwarf.
Highlights
White dwarf stars are routinely used to constrain the age and past history of the Galactic populations, including the solar neighborhood and open and globular clusters (Von Hippel & Gilmore 2000; Hansen et al 2007; Winget et al 2009; García-Berro et al 2010; Bono et al 2013, and references therein)
Nine sequences span the range of masses corresponding to extremely low-mass (ELM) white dwarfs (M 0.20 M )
Our grid of He-core white dwarf sequences is appropriate for homogeneous mass and cooling age determinations of observed low-mass white dwarfs for all evolutionary stages where these stars have chances of being observed: the final cooling branch, the stages at constant luminosity following the end of Roche lobe overflow, and the evolutionary stages prior to the occurrence of CNO flashes on the early cooling branches
Summary
White dwarf stars are routinely used to constrain the age and past history of the Galactic populations, including the solar neighborhood and open and globular clusters (Von Hippel & Gilmore 2000; Hansen et al 2007; Winget et al 2009; García-Berro et al 2010; Bono et al 2013, and references therein). The development of a fine and homogeneous grid of evolutionary sequences for He-core white dwarfs and especially ELM white dwarfs, which are derived consistently from binary evolution, is a pressing necessity for precise mass and cooling age determinations, and asteroseismological inferences for these stars This is the main aim of this paper. We show in particular that seismic tools exploiting these properties can be used to extract information about the occurrence of CNO flashes in prior stages and the consequent age dichotomy expected in low-mass He-core white dwarfs Our interest in this aspect is motivated by the fact that the three pulsating ELM white dwarfs discovered by Hermes et al (2013) have stellar mass near the threshold value for the occurrence of CNO flashes.
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