Axions and the Cooling of White Dwarf Stars

Bertolami

Córsico

2013

A&A

256

475

Context. The number of detected extremely low-mass (ELM) white dwarf stars has increased drastically in recent years, thanks to the results of many surveys. In addition, some of these stars have been found to exhibit pulsations, making them potential targets for asteroseismology. Aims. We provide a fine and homogeneous grid of evolutionary sequences for helium (He) core white dwarfs for the whole range of their expected masses (0.15 M * /M 0.45), including the mass range for ELM white dwarfs (M * /M 0.20). The grid is appropriate for mass and age determination of these stars, as well as for studying their adiatabic pulsational properties. Methods. 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. Realistic initial models for the evolving white dwarfs have been obtained by computing the nonconservative evolution of a binary system consisting of an initially 1 M ZAMS star and a 1.4 M neutron star for various initial orbital periods. To derive cooling ages and masses for He-core white dwarfs, we perform a least square fitting of the M(T eff , g) and Age(T eff , g) relations provided by our sequences by using a scheme that takes into account the time spent by models in different regions of the T eff − g plane. This is particularly useful when multiple solutions for cooling age and mass determinations are possible in the case of CNO-flashing sequences. We also explore in a preliminary way the adiabatic pulsational properties of models near the critical mass for the development of CNO flashes (∼0.2 M ). This is motivated by the discovery of pulsating white dwarfs with stellar masses near this threshold value. Results. We obtain reliable and homogeneous mass and cooling age determinations for 58 very low-mass white dwarfs, including three pulsating stars. Also, we find substantial differences in the period spacing distributions of g-modes for models with stellar masses near ∼0.2 M , 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 the masses and ages of He-core white dwarfs.

Section: Introductionmentioning

confidence: 99%

New evolutionary sequences for extremely low-mass white dwarfs

Bertolami

Córsico

2013

A&A

256

475

“…A detailed assessment of the accuracy of current estimates of white dwarf cooling rates is therefore a pressing necessity, for carbonoxygen white dwarfs are increasingly being employed to constrain the age and past history of Galactic populations, including Tables 1 to 4, the equation of state, boundary conditions, and CO phase diagram routines are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/555/A96 Appendices A and B are available in electronic form at http://www.aanda.org the solar neighbourhood, open and globular clusters (see, i.e., Winget et al , 2009García-Berro et al 1988Hansen et al 2007;Bedin et al 2008Bedin et al , 2010, and references therein). Furthermore, theoretical estimates of white dwarf cooling rates are routinely adopted to place constraints on the properties of neutrinos, exotic particles, and dark matter candidates (Freese 1984;Isern et al 1992Isern et al , 2008Winget et al 2004;Bertone & Fairbairn 2008;Córsico et al 2012) and alternative theories of gravity (see, e.g., Garcia-Berro et al 1995, 2011Benvenuto et al 2004). These types of investigations all demand an accurate calculation of white dwarf cooling models.…”

Section: Introductionmentioning

confidence: 99%

Comparison of theoretical white dwarf cooling timescales

Salaris

Garcı́a–Berro

2013

A&A

Self Cite

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. 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. 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 white dwarf models with both pure carbon and uniform carbon-oxygen (50/50 mass fractions) cores, and pure hydrogen layers with mass fraction q H = 10 −4 M WD on top of pure helium buffers of mass q He = 10 −2 M WD . 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 ) > −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, 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 ) ∼ −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.

“…Additionally, they can be used to place constraints on exotic elementary particles (Isern et al 1992;Córsico et al 2001;Isern et al 2008) or on alternative theories of gravitation (García-Berro et al 1995;Benvenuto et al 2004;García-Berro et al 2011). This is possible because we have a relatively precise knowledge of the main physical processes responsible for their evolution, although some uncertainties still persist for key aspects of their constitutive physics.…”

Section: Introductionmentioning

confidence: 99%

New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution

Garcı́a–Berro

Isern

et al. 2011

A&A

Self Cite

Context. Cool white dwarfs are reliable and independent stellar chronometers. The most common white dwarfs have carbon-oxygen dense cores. Consequently, the cooling ages of very cool white dwarfs sensitively depend on the adopted phase diagram of the carbonoxygen binary mixture. Aims. A new phase diagram of dense carbon-oxygen mixtures appropriate for white dwarf interiors has been recently obtained using direct molecular dynamics simulations. In this paper, we explore the consequences of this phase diagram in the evolution of cool white dwarfs. Methods. To do this we employ a detailed stellar evolutionary code and accurate initial white dwarf configurations, derived from the full evolution of progenitor stars. We use two different phase diagrams, that of Horowitz et al. (2010, Phys. Rev. Lett., 104, 231101), which presents an azeotrope, and the phase diagram of Segretain & Chabrier (1993, A&A, 271, L13), which is of the spindle form. Results. We computed the evolution of 0.593 and 0.878 M white dwarf models during the crystallization phase, and we found that the energy released by carbon-oxygen phase separation is smaller when the new phase diagram of Horowitz et al. is used. This translates into time delays that are on average a factor ∼2 smaller than those obtained when the phase diagram of Segretain & Chabrier is employed. Conclusions. Our results have important implications for white dwarf cosmochronology, because the cooling ages of very old white dwarfs are different for the two phase diagrams. This may have a noticeable impact on the age determinations of very old globular clusters, for which the white dwarf color-magnitude diagram provides an independent way of estimating their age.