The white dwarf cooling age of NGC 6791

GARCÍA-BERRO, E.; ALTHAUS, L. G.; TORRES, S.; LOREN-AGUILAR, P.; RENEDO, I.; CÓRSICO, A. H.; ISERN, J.; SALARIS, M.; ROHRMANN, R. D.

Tübingen

Workshop; 17th European White Dwarf Workshop; 2010

NGC 6791 is a metal-rich ([Fe/H]=+0.4), well populated (3,000 stars) and nearby (m − M = 13.44mag ) open cluster that has been imaged down to luminosities below those of the faintest white dwarfs, allowing us to obtain a reliable white dwarf luminosity function. The main-sequence turn-off age (8 Gyr) and the age derived from the termination of the white dwarf cooling sequence (6 Gyr) are very different. One possible explanation is that as white dwarfs cool, one of the ashes of helium burning, 22 Ne, sinks in the deep interior of these  stars. At lower temperatures, white dwarfs are expected to crystallize and phase separation of 12 C and 16 O is expected to occur. This sequence of events is expected to introduce significant delays in the cooling times. Using the white dwarf luminosity function of the metal-rich galactic cluster NGC 6791, we present the first solid observational evidence that indeed both gravitational sedimentation  and phase separation upon crystallization occur in white dwarfs.