Comparison of theoretical white dwarf cooling timescales.

Context. An accurate assessment of white dwarf cooling times is
paramount so that white dwarf cosmochronology of Galactic populations
can be put on more solid grounds. This issue is particularly relevant in
view of the enhanced observational capabilities provided by the next
generation of extremely large telescopes, that will offer more avenues
to use white dwarfs as probes of Galactic evolution and test-beds of
fundamental physics. <br /> Aims: We estimate for the first time the
consistency of results obtained from independent evolutionary codes for
white dwarf models with fixed mass and chemical stratification, when the
same input physics is employed in the calculations. <br /> Methods: We
compute and compare cooling times obtained from two independent and
widely used stellar evolution codes, BaSTI and LPCODE evolutionary
codes, using exactly the same input physics for 0.55 M<sub>⊙</sub>
white dwarf models with both pure carbon and uniform carbon-oxygen
(50/50 mass fractions) cores, and pure hydrogen layers with mass
fraction q<sub>H</sub> = 10<sup>-4</sup>M<sub>WD</sub> on top of pure
helium buffers of mass q<sub>He</sub> = 10<sup>-2</sup>M<sub>WD</sub>.
<br /> Results: Using the same radiative and conductive opacities,
photospheric boundary conditions, neutrino energy loss rates, and
equation of state, cooling times from the two codes agree within ~2% at
all luminosities, except when log (L/L<sub>⊙</sub>) > -1.5 where
differences up to ~8% do appear, because of the different thermal
structures of the first white dwarf converged models at the beginning of
the cooling sequence. This agreement is true for both pure carbon and
uniform carbon-oxygen stratification core models, and also when the
release of latent heat and carbon-oxygen phase separation are
considered. We have also determined quantitatively and explained the
effect of varying equation of state, low-temperature radiative
opacities, and electron conduction opacities in our calculations, <br />
Conclusions: We have assessed for the first time the maximum possible
accuracy in the current estimates of white dwarf cooling times,
resulting only from the different implementations of the stellar
evolution equations and homogeneous input physics in two independent
stellar evolution codes. This accuracy amounts to ~2% at luminosities
lower than log (L/L<sub>⊙</sub>) ~ -1.5. This difference is smaller
than the uncertainties in cooling times attributable to the present
uncertainties in the white dwarf chemical stratification. Finally, we
extend the scope of our work by providing tabulations of our cooling
sequences and the required input physics that can be used as a
comparison test of cooling times obtained from other white dwarf
evolutionary codes.Fil: Salaris, M.. Astrophysics Research Institute. Liverpool John Moores University. Liverpool Science Park; Reino Unido;Fil: Althaus, Leandro Gabriel. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Grupo de Evolución Estelar y Pulsaciones; Argentina; Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - La Plata. Instituto de Astrofisica la Plata; Argentina;Fil: García-Berro, E.. Departament de Física Aplicada. Universitat Politècnica de Catalunya; Argentina; Institute for Space Studies of Catalonia; Argentina