Volume 537, January 2012
|Number of page(s)||7|
|Section||Stellar structure and evolution|
|Published online||23 December 2011|
New phase diagrams for dense carbon-oxygen mixtures and white dwarf evolution
1 Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
2 Member of CONICET, Argentina
e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org;
3 Departament de Física Aplicada, Universitat Politècnica de Catalunya, c/Esteve Terrades 5, 08860 Castelldefels, Spain
4 Institute for Space Studies of Catalonia, c/Gran Capità 2–4, Edif. Nexus 104, 08034 Barcelona, Spain
5 Institut de Ciències de l’Espai (CSIC), Facultat de Ciències, Campus UAB, Torre C5-parell, 08193 Bellaterra, Spain
Received: 17 August 2011
Accepted: 19 October 2011
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 carbon-oxygen 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.
Key words: stars: evolution / white dwarfs / stars: interiors
© ESO, 2012
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