THE COOLING OF OXYGEN-NEON WHITE-DWARFS

Recent observations of very cool white dwarfs suggest that the low-lum inosity portion of the white dwarf luminosity function is indeed domin ated by the contribution of massive objects. These objects should pred ominantly be made of oxygen, neon and other trace isotopes. In this pa per we compute cooling sequences for massive oxygen-neon (ONe) white d warfs and we examine the question of whether massive ONe white dwarfs could represent a significant contribution to the white dwarf luminosi ty function at very low luminosities. We fmd that massive ONe white dw arfs cool much faster than carbon-oxygen (CO) white dwarfs and that, c onsequently, their population effectively detaches from the bulk of CO white dwarfs leading to the formation of a tail in the low-luminosity portion of the white dwarf luminosity function. Nevertheless, the pos ition of the reported cut-off in the white dwarf luminosity function i s not affected and, therefore, the age estimates of the solar neighbou rhood inferred from the position of the drop-off of the white dwarf lu minosity function remain unaffected.