Crystallization of carbon-oxygen mixtures in white dwarfs

The phase diagram of the carbon-oxygen dense plasma is computed, using a density functional approach. The two nuclear species are found to be miscible in solid phase, in contradiction with a previous study by Stevenson (1980). The phase diagram is of the "spindle" form, the solid phase being slightly more oxygen-rich than the fluid phase. The astrophysical consequences of this result are discussed, with a special attention to the white dwarf luminosity function. The release of gravitational energy in the white dwarf interior due to the change of composition at crystallization is moderate. The determination of the age of the galactic disk from the white dwarf luminosity function is therefore only weakly affected, compared to the case of complete chemical separation implied by Stevenson's phase diagram.