Theoretical white-dwarf luminosity functions for two phase diagrams of the carbon-oxygen dense plasma

Summary. The white dwarf luminosity function is obtained for two phase diagrams of the carbon-oxygen dense plasma. In the first case, carbon and oxygen are totally mixed in solid phase and we obtain results that are in good agreement with recent calculations of Iben and Tutukov (1984). In the second case, either pure carbon or pure oxygen crystallizes, depending on the composition of the liquid mixture compared to an eutectic having a carbon mass fraction XC 0.6. Due to the larger density of solid oxygen, this leads to the formation of an oxygen core surrounded by a carbon rich mantle. The redistribution of elements releases as substantial amount of energy which slows down the cooling. The resulting theoretical luminosity function falls abruptly at log (L/L☉) - 4.6, this value corresponding to a cooling time of 15 Gyr, assumed to be the age of the galactic disk. Carbon-oxygen separation can therefore explain the low luminosity cut-off observed in the white dwarf luminosity function which is not predicted by standard cooling theory. © European Southern Observatory.