A Large Stellar Evolution Database for Population Synthesis Studies. Vi. White Dwarf Cooling Sequences

Abstract

We present a new set of cooling models and isochrones for both H-and Heatmosphere white dwarfs, incorporating accurate boundary conditions from detailed model atmosphere calculations, and carbon-oxygen chemical abundance profiles based on updated stellar evolution calculations from the BaSTI stellar evolution archive -a theoretical data center for the Virtual Observatory. We discuss and quantify the uncertainties in the cooling times predicted by the models, arising from the treatment of mixing during the central H-and He-burning phases, number of thermal pulses experienced by the progenitors, progenitor metallicity and the 12 C(α, γ) 16 O reaction rate. The largest sources of uncertainty turn out to be related to the treatment of convection during the last stages of the progenitor central He-burning phase, and the 12 C(α, γ) 16 O reaction rate.We compare our new models to previous calculations performed with the same stellar evolution code, and discuss their application to the estimate of the age of the solar neighborhood, and the interpretation of the observed number ratios between H-and He-atmosphere white dwarfs. The new white dwarf sequences and an extensive set of white dwarf isochrones that cover a large range of ages and progenitor metallicities are made publicly available at the official BaSTI website.

A large oxygen-dominated core from the seismic cartography of a pulsating white dwarf

“…We find that the central abundance of oxygen rises up to 86.2% (± 4%), that is, 15% more than is predicted from evolutionary calculations 2 . The extent of the homogeneous central part of the core is nearly doubled, reaching a fractional mass depth of log(q) ≈ − 0.7 (or 0.45 solar masses, M  ), instead of log(q) ≈ − 0.35 (0.32M  ).…”

Hubble Space Telescope observations of the Kepler-field cluster NGC 6819 – I. The bottom of the white dwarf cooling sequence★

“…WD isochrones (for both DA and DB objects) with the same age-distance-reddening combinations are displayed in the lower panel of the same figure. Notice how the termination of the DB sequence is brighter than that for the DA WDs, contrary to the result for typical globular cluster ages, because in this regime He-envelope WDs cool down more slowly that the H-envelope counterparts (see, e.g., Salaris et al 2010). To compare WD and MS TO ages we have used the completeness corrected WD LF, and matched with theoretical WD LFs of varying age the observed cut-off of the star counts.…”

“…The corresponding mass and magnitudes of the synthetic WD are then determined by quadratic interpolation along the isochrone, that has been calculated assuming a WD initial-final mass relation (see Salaris et al 2010 for details about the WD isochrones) 4 . For the fraction of objects assumed to be in unresolved binaries, we extract randomly also a value for the ratio q between the initial mass of the companion and the mass of the WD progenitor, according to a specified statistical distribution.…”

“…Panel b) shows the effect of including a 15% fraction of DB objects (dashed line -DB isochrones are also from Salaris et al 2010) for an age of 2.25 Gyr, and a reference 22% binary progenitor fraction. Also in this case the magnitude of the LF cut off is unchanged, and the signature of DB WDs appears at mF606W ∼25.5 (the variation of the star counts at fainter magnitudes is again a consequence of keeping the total number of objects fixed).…”

“…To this purpose we display the LF for the standard Salpeter MF (solid line -no DBs, 22% of WD+WD unresolved binaries) and a LF calculated with a close to flat MF for the WD progenitors (dashed line), i.e., a power law with exponent −0.15. Progenitor masses are between ∼1.7 and ∼6.5 M⊙ at our reference age of 2.25 Gyr, whilst the final WD masses range between ∼0.61 and 1.0 M⊙ according to the initial-final mass relation adopted by the BaSTI WD isochrones (see Salaris et al 2010 for details).…”