Abstract
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.
Highlights
White dwarfs are the evolutionary remnant of stars of intermediate and low masses at the zero-age main sequence
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 these purposes, and that its shape function is almost insensitive to the most relevant inputs, such as the adopted cooling sequences, the initial mass function, the density profile of the stellar spheroid, or the adopted fraction of unresolved binaries
We compare the results of our simulations to the halo luminosity function of Rowell & Hambly (2011), and we study the sensitivity of the theoretical white dwarf luminosity function to different model inputs
Summary
White dwarfs are the evolutionary remnant of stars of intermediate and low masses at the zero-age main sequence. These cooling tracks improve upon those used in the early and pioneering calculations of Isern et al (1998) and García-Berro et al (2004), and in the more recent calculation of van Oirschot et al (2014) These evolutionary sequences have self-consistently evolved from the zero-age main sequence, through the red giant and thermally pulsing AGB phases to the white dwarf regime, and have revealed the important role of residual hydrogen burning in the atmospheres of low-mass white dwarfs, a physical process that needs verification.
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