A Study of Cool White Dwarfs in the Sloan Digital Sky Survey Data Release 12

TL;DR

This study analyzes a large sample of white dwarfs from SDSS DR12 and Gaia DR2 to understand their cooling, formation channels, and spectral type distributions, revealing insights into their mass, temperature, and evolutionary processes.

Contribution

It provides the first comprehensive comparison of DAs and non-DAs in a large, combined SDSS and Gaia dataset, including mass and temperature distributions and their evolutionary implications.

Findings

DCs are about 0.15 solar masses more massive than DAs and DBs.

The non-DA to DA ratio increases significantly below 15,000 K.

Approximately 14% of DAs may convert to non-DAs due to convective dilution.

Abstract

In this work we study white dwarfs where $30\,000\,\text{K} {>} \mathrm{T}_{\rm{eff}} {>} 5\,000\,\text{K}$ 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 \textit{Gaia} survey DR2. We present the mass distribution for DAs, DBs and DCs, where it is found that the DCs are ${\sim}0.15\,\mathrm{M}_\odot$ 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 ${\sim}0.075$ for effective temperature above ${\sim}22\,000\,\text{K}$ and increases by a factor of five for effective temperature cooler than $15\,000\,\text{K}$. If we assume that the increase of non-DA stars between ${\sim}22\,000\,\text{K}$ to ${\sim}15\,000\,\text{K}$ is due to convective dilution, $14{\pm}3$ 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.