The Population of Halo White Dwarfs

Abstract

The discovery of microlenses towards the Large Magellanic Cloud has generated a large controversy about the possibility that white dwarfs could be responsible for these microlensing events and, thus, could provide a significant contribution to the mass budget of our Galactic halo. However, white dwarfs as viable dark matter candidates are not free of problems, since an excess of them would imply as well an overproduction of red dwarfs and Type II Supernovae. The debate of whether or not white dwarfs contribute significantly to the Galactic halo dark matter has motivated a large number of observational searches and theoretical works and is still open. Among the observational surveys perhaps the most extensive one is that of Oppenheimer et al. (2001) who discovered 38 faint white dwarfs with large proper motions in digitized photographic plates from the SuperCOSMOS Sky Survey. The interpretation of the old, cool white dwarfs of this sample is still controversial. Whereas these authors claim that they have finally found the elusive ancient halo white dwarf population that contributes significantly to the mass budget of the galactic halo, there have been several other contributions that argue that these white dwarfs are not genuine halo members but, instead, thick disk stars. We show here by making use of our Monte Carlo simulator (Garcia-Berro et al. 1999; Torres et al. 1998) that the interpretation of this sample is based on the adopted distances, which are obtained from a color-magnitude calibration, and we demonstrate that when the correct distances are used a sizeable fraction of these putative halo white dwarfs belong indeed to the disk population.