Revisiting the luminosity function of single halo white dwarfs

Abstract

White dwarfs are the fossils left by the evolution of low-andintermediate-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 theluminosity function of halo white dwarfs, aimed at investigating whichinformation can be derived from the currently available observed data.We employ an up-to-date population synthesis code based on Monte Carlotechniques, that incorporates the most recent and reliable coolingsequences for metal poor progenitors as well as an accurate modeling ofthe observational biases. We find that because the observed sample ofhalo white dwarfs is restricted to the brightest stars only the hotbranch of the white dwarf luminosity function can be used for suchpurposes, and that its shape function is almost insensitive to the mostrelevant inputs, like the adopted cooling sequences, the initial massfunction, the density profile of the stellar spheroid, or the adoptedfraction of unresolved binaries. Moreover, since the cut-off of theobserved luminosity has not been yet determined only lower limits to theage of the halo population can be placed. We conclude that the currentobserved sample of the halo white dwarf population is still too small toobtain definite conclusions about the properties of the stellar halo,and the recently computed white dwarf cooling sequences whichincorporate residual hydrogen burning should be assessed usingmetal-poor globular clusters.