On the origin of the ultramassive white dwarf GD50

We argue on the basis of astrometric and spectroscopic data that the ultramassive white dwarf GD50 is associated with the star formation event that created the Pleiades and is possibly a former member of this cluster. Its cooling age (similar to 60 Myr) is consistent with it having evolved essentially as a single star from a progenitor with a mass M > 6M circle dot, so we find no need to invoke a white dwarf-white dwarf binary merger scenario to account for its existence. This result may represent the first direct observational evidence that single-star evolution can produce white dwarfs with M > 1.1M circle dot, as predicted by some stellar evolutionary theories. On the basis of its tangential velocity, we also provisionally identify the ultramassive (M similar to 1.2M circle dot) white dwarf PG 0136 + 251 as being related to the Pleiades. These findings may help to alleviate the difficulties in reconciling the observed number of hot nearby ultramassive white dwarfs with the smaller number predicted by binary evolution models under the assumption that they are the products of white dwarf mergers.