New Conductive Opacities for White Dwarf Envelopes

Abstract

Thanks to their continuous cooling and relative simplicity, white dwarf stars are routinely used to measure the ages of stellar populations. The usefulness of white dwarfs as cosmochronometers depends on the availability of accurate cooling models. A key ingredient of those models is conductive opacities, which largely govern the cooling rate. In this work, we present improved conductive opacities for the regime of moderate coupling and moderate degeneracy that characterizes an important portion of the envelopes of DA and DB white dwarfs. We find differences of up to a factor of 3 between our calculations and the commonly used opacities of Cassisi et al., which we attribute to an improved account of electron–electron scattering. The cooling models are strongly affected by those changes in the conductive opacities: the age of a 4000 K white dwarf can be reduced by as much as 2 Gyr. We provide analytical fits to our new opacities to facilitate the implementation of this important effect in white dwarf evolution codes.

Toward precision cosmochronology

“…The magnitude of the crystallization pile-up is very similar, and the fit to the low-luminosity cutoff is nearly identical. We note that the corrections to the Cassisi et al (2007) opacities in the regime of moderate Coulomb coupling and partial electron degeneracy (Blouin et al 2020), which significantly increase the conductivity of the H envelope, were crucial to obtaining a cutoff close to the observations. Tremblay et al (2019) were able to reproduce the cutoff without these corrections because they were relying on older calculations (Hubbard & Lampe 1969;Itoh et al 1983;Mitake et al 1984) that predict higher conductivities than Cassisi et al (2007) in the core and in the He envelope (Salaris et al 2013).…”

“…To do so we used STELUM, the Montreal white dwarf evolution code (Brassard & Fontaine 2018). The constitutive physics is identical to that described in Fontaine et al (2001), except that (1) we now use the Cassisi et al (2007) conductive opacities, which we correct in the moderately coupled and moderately degenerate regime (Blouin et al 2020) following the new theory of ; (2) the plasma coupling parameter at the phase transition is given by our new C/O phase diagram;…”

The updated basti stellar evolution models and isochrones – III. White dwarfs

“…Hansen 1998 ;Saumon & Jacobson 1999 ). Mass-radius relations and CMDs for all models calculated with Blouin et al ( 2020 ) opacities are identical to those displayed in these figures.…”

“…For each mass and core chemical stratification, we have computed models for two different choices of the electron conduction opacities -from Cassisi et al ( 2007 ) and Blouin et al ( 2020 ), respectivelyas described in Section 2.1. Figs 1 and 2 display, as an example, the radius of our grid of H-atmosphere and He-atmosphere models calculated with Cassisi et al ( 2007 ) opacities as a function of the ef fecti ve temperature T eff , and the resulting cooling tracks in selected colour magnitude diagrams (CMDs).…”

“…Electron conduction opacities are from the calculations by Cassisi et al ( 2007 ) in one set of models, and from Blouin et al ( 2020 ) for the H and He envelopes (keeping Cassisi et al 2007 , opacities for the CO cores) in a second set. Blouin et al ( 2020 ) have recently published new, impro v ed calculations of electron conduction opacities for H and He compositions for moderate de generac y (i.e. when θ ∼ 1, where θ ≡ T / T F , and T F is the Fermi temperature) which they have bridged with Cassisi et al ( 2007 ) calculations for strong de generac y ( θ ࣠ 0.1).…”