Revisiting the luminosity function of single halo white dwarfs
  White dwarfs are the fossils left by the evolution of low-and
intermediate-mass stars, and have very long evolutionary timescales. This
allows us to use them to explore the properties of old populations, like the
Galactic halo. We present a population synthesis study of the luminosity
function of halo white dwarfs, aimed at investigating which information can be
derived from the currently available observed data. We employ an up-to-date
population synthesis code based on Monte Carlo techniques, that incorporates
the most recent and reliable cooling sequences for metal poor progenitors as
well as an accurate modeling of the observational biases. We find that because
the observed sample of halo white dwarfs is restricted to the brightest stars
only the hot branch of the white dwarf luminosity function can be used for such
purposes, and that its shape function is almost insensitive to the most
relevant inputs, like the adopted cooling sequences, the initial mass function,
the density profile of the stellar spheroid, or the adopted fraction of
unresolved binaries. Moreover, since the cut-off of the observed luminosity has
not been yet determined only lower limits to the age of the halo population can
be placed. We conclude that the current observed sample of the halo white dwarf
population is still too small to obtain definite conclusions about the
properties of the stellar halo, and the recently computed white dwarf cooling
sequences which incorporate residual hydrogen burning should be assessed using
metal-poor globular clusters.
