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Keywords:

  • binaries: general;
  • stars: evolution;
  • white dwarfs;
  • Galaxy: kinematics and dynamics;
  • solar neighbourhood;
  • Galaxy: stellar content

ABSTRACT

We present a new synthetic population model for local thin-disc (d≤ 100 pc) single-star white dwarfs (WDs), including effective temperature and mass distribution. The only two parameters of the synthetic population model are the initial mass function (IMF) and the star-formation rate (SFR). Depletion losses through kinematic heating of the stellar ‘gas’ vertical to the Galactic plane are prescribed empirically using the observed local velocity dispersion as a function of age. We apply the same SFR and IMF for the WD population model as previously determined from a study based upon the latest Hipparcos and binary catalogue data to yield a matching synthetic single-star population.

A striking result of comparing the synthetic WD population with the complete local observed sample (with d < 13 pc) is the excellent agreement between the absolute number of WDs when binary stars are not excluded from the empirical basis used to calibrate our synthetic population. When looking at the total expected WD number after a rigorous accounting for binary stars, we see that this is significantly lower than the corresponding observed WD number. Hence, many of these apparently single WDs must have a hidden binary-system history, i.e. some may be end products of binary mergers or of mass overflows. We suspect that many of the WDs exist in hidden double-degenerate systems.

There is good agreement between the temperature distribution of our synthetic WD sample and observations, as well as between our synthetic mass-distribution peak (at 0.67 M) and the one recently observed (0.65 M). Remarkably, both values are about 0.06 M higher than those stated by earlier studies. In the case of our synthetic sample, older stars of lower mass experience a greater amount of dynamic depletion and the remaining local WDs within the sample tend to be slightly more massive. The small, remaining discrepancy may be explained by the stated contamination by WDs of binary origin.