The Gaia DR2 halo white dwarf population: the luminosity function, mass distribution, and its star formation history

Abstract

We analyse the volume-limited nearly complete 100 pc sample of 95 halo white dwarf candidates identified by the second data release of Gaia. Based on a detailed population synthesis model, we apply a method that relies on Gaia astrometry and photometry to accurately derive the individual white dwarf parameters. This method is tested with 25 white dwarfs of our sample for which we took optical spectra and performed spectroscopic analysis. We build and analyse the halo white dwarf luminosity function, for which we find for the first time possible evidences of the cut-off, leading to an age estimate of ${\simeq}12\pm 0.5$ Gyr. The mass distribution of the sample peaks at $0.589\, \mathrm{ M}_{\odot }$, with $71{{\ \rm per\ cent}}$ of the white dwarf masses below $0.6\, \mathrm{ M}_{\odot }$ and just two massive white dwarfs of more than $0.8\, \mathrm{ M}_{\odot }$. From the age distribution, we find three white dwarfs with total ages above 12 Gyr, of which J1312−4728 is the oldest white dwarf known with an age of $12.41\pm 0.22\,$ Gyr. We prove that the star formation history is mainly characterized by a burst of star formation that occurred from 10 to 12 Gyr in the past, but extended up to 8 Gyr. We also find that the peak of the star formation history is centred at around 11 Gyr, which is compatible with the current age of the Gaia-Enceladus encounter. Finally, $13{{\ \rm per\ cent}}$ of our halo sample is contaminated by high-speed young objects (total age <7 Gyr). The origin of these white dwarfs is unclear but their age distribution may be compatible with the encounter with the Sagittarius galaxy.

A catalogue of white dwarfs in Gaia EDR3

“…Bergeron et al 2019;Coutu et al 2019;Ourique et al 2019;Tremblay et al 2019a;Chandra et al 2020). The white dwarf luminosity function was also reexplored with unprecedented level of detail (Torres et al 2021); and Torres et al (2019) further investigated the memberships of white dwarfs into the thin disc, thick disc, and halo Galactic populations. Significant progress was also made for largescale identification and characterization of white dwarfs in binaries with main-sequence stars, either in common proper motion pairs (El-Badry, Rix & Weisz 2018), non-interacting unresolved systems (Inight et al 2021), or cataclysmic variables (Abril et al 2020;Pala et al 2020).…”

How to search for Mirror Stars with Gaia

“…Given the comparison, it seems likely that these mirror star candidates are simply particularly dim white dwarfs. In favour of this hypothesis, we note that their location in the colour-magnitude diagram would be very close to the region populated by old and massive helium-atmosphere white dwarfs [68] with a small amount of dust extinction. Fig.…”

Constraining the solar neighbourhood age-metallicity relation from white dwarf-main sequence binaries

“…For the Gaia population, we are analysing objects with average distances of 150 pc (Section 2), which defines a sample close to be volume-limited. In a volume-limited sample not only hot WDs (>10,000 K) are intrinsically less numerous but also the completeness of cold (<5000 K) and less luminous WDs decreases to 60 per cent (Torres et al 2021). These effects imply that, overall, it becomes intrinsically more difficult to find WDs with ages longer than 10 Gyrs.…”