A study of cool white dwarfs in the Sloan Digital Sky Survey Data Release 12

Abstract

In this work we study white dwarfs where 30 000 K>T eff >5 000 K to compare the differences in the cooling of DAs and non-DAs and their formation channels. Our final sample is composed by nearly 13 000 DAs and more than 3 000 non-DAs that are simultaneously in the SDSS DR12 spectroscopic database and in the Gaia survey DR2. We present the mass distribution for DAs, DBs and DCs, where it is found that the DCs are ∼0.15 M more massive than DAs and DBs on average. Also we present the photometric effective temperature distribution for each spectral type and the distance distribution for DAs and non-DAs. In addition, we study the ratio of non-DAs to DAs as a function of effective temperature. We find that this ratio is around ∼0.075 for effective temperature above ∼22 000 K and increases by a factor of five for effective temperature cooler than 15 000 K. If we assume that the increase of non-DA stars between ∼22 000 K to ∼15 000 K is due to convective dilution, 14±3 per cent of the DAs should turn into non-DAs to explain the observed ratio. Our determination of the mass distribution of DCs also agrees with the theory that convective dilution and mixing are more likely to occur in massive white dwarfs, which supports evolutionary models and observations suggesting that higher mass white dwarfs have thinner hydrogen layers.

Evidence of spectral evolution on the white dwarf sample from the Gaia mission

“…Our results also indicate that more than 80 per cent of all observed helium envelope white dwarfs, i.e. coming from spectral evolution or by primordial hydrogen deficiency, are below 12 000, where they do not show helium lines because of the low temperature, in agreement with Ourique et al (2019) results.…”

“…The dependence of the spectral evolution on the effective temperature is expected since it is directly related to convection episodes during the white dwarf evolution. Kepler et al (2019) show that the mass distribution for DBs has a mean of 0.618±0.004 M , assuming pure Helium atmospheres, with a very small number of DBs at higher masses, in agreement with Genest-Beaulieu & Bergeron (2019) and Ourique et al (2019). This is an indication that the process that results in extremely thin -or non-existent -hydrogen layer white dwarfs (probably Late Thermal Pulse, Very Late Thermal Pulse, or merger) is not effective for the progenitors of white dwarfs with masses higher than 0.75 M .…”

“…The existence of DBAs and DABs, i.e., white dwarfs with helium and hydrogen lines on its spectra, is a evidence that mixing is not necessarily instantaneous (Bergeron et al 1997;Chen & Hansen 2011;Rolland et al 2018;Ourique et al 2019). However, since we do not know the duration of these events, our approximation is that the mixing duration is much smaller than the evolutionary time scale.…”

“…Rolland et al (2018) propose that spectral evolution could be driven by convective mixing, which should occur for effective temperatures lower than 13 000 K and hydrogen mass between log (MH/M * )∼10 −15 and 10 −6 . Ourique et al (2019) studied the number of spectroscopic DAs and non-DAs from Kepler et al (2016) and Munn et al (2017) and found that the helium to hydrogen white dwarf number ratio goes from ∼0.075 for effective temperatures around 30 000 K to ∼0.36 for effective temperatures around 11 000 K and becomes nearly constant for colder temperatures. From this results they conclude that most non-DAs are the result of the spectral evolution, since the very late thermal pulse, which produces helium envelope white dwarfs, is predicted not to occur for more than 0.16 − 0.25 of all white dwarfs (Blöcker 2001;Lawlor & MacDonald 2006).…”

From hydrogen to helium: the spectral evolution of white dwarfs as evidence for convective mixing

“…In contrast, larger distances may be able to overcome low number statistics for spectroscopic ratios, without necessarily adding more contaminants. The largest volume results in an approach that is closer to those of Genest-Beaulieu & Bergeron (2019b) and Ourique et al (2019).…”