The ages of very cool hydrogen-rich white dwarfs

The evolution of white dwarfs is essentially a cooling process that depends primarily on the energy stored in their degenerate cores and on the transp arency of their envelopes. In this pager we compute accurate cooling sequen ces for carbon-oxygen white dwarfs with hydrogen dominated atmospheres for the full range of masses of interest. For this purpose we use the most accu rate available physical inputs for both the equation of state and opacities of the envelope and for the thermodynamic quantities of the degenerate cor e. We also investigate the role of the latent heat in the computed cooling sequences. We present separately cooling sequences in which the effects of phase separation of the carbon-oxygen binary mixture upon crystallization h ave been neglected, and the delay introduced in the cooling times when this mechanism is properly taken into account, in order to compare our results with other published cooling sequences which do not include a treatment of this phenomenon. We find that the cooling ages of very cool white dwarfs wi th pure hydrogen atmospheres have been systematically underestimated by rou ghly 1.5 Gyr at log (L/L.) = -4.5 for an otherwise typical similar to0.6 M. white dwarf, when phase separation is neglected. If phase separation of th e binary mixture is included, then the cooling ages are further increased b y roughly 10%. Cooling tracks and cooling isochrones in several color-magni tude diagrams are presented as well.