We report the discovery of 6 576 new spectroscopically confirmed white dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 12. We obtain T eff , log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found one central star of a planetary nebula, one ultra-compact helium binary (AM CVn), one oxygen line dominated white dwarf, 15 hot DO/PG1159s, 12 new cataclysmic variables, 36 magnetic white dwarf stars, 54 DQs, 115 helium dominated white dwarfs, 148 white dwarf+main sequence star binaries, 236 metal polluted white dwarfs, 300 continuum spectra DCs, 230 hot subdwarfs, 2 936 new hydrogen dominated white dwarf stars, and 2 675 cool hydrogen dominated subdwarf stars. We calculate the mass distribution of all 5883 DAs with S/N 15 in DR12, including the ones in DR7 and DR10, with an average S/N=26, corrected to the 3D convection scale, and also the distribution after correcting for the observed volume, using 1/V max .

We report the discovery of 9 089 new spectroscopically confirmed white dwarfs and subdwarfs in the Sloan Digital Sky Survey Data Release 10. We obtain T eff , log g and mass for hydrogen atmosphere white dwarf stars (DAs) and helium atmosphere white dwarf stars (DBs), and estimate the calcium/helium abundances for the white dwarf stars with metallic lines (DZs) and carbon/helium for carbon dominated spectra DQs. We found 1 central star of a planetary nebula, 2 new oxygen spectra on helium atmosphere white dwarfs, 71 DQs, 42 hot DO/PG1159s, 171 white dwarf+main sequence star binaries, 206 magnetic DAHs, 327 continuum dominated DCs, 397 metal polluted white dwarfs, 450 helium dominated white dwarfs, 647 subdwarfs and 6888 new hydrogen dominated white dwarf stars.

White dwarfs carry information on the structure and evolution of the Galaxy, especially through their luminosity function and initial-to-final mass relation. Very cool white dwarfs provide insight into the early ages of each population. Examining the spectra of all stars with 3σ proper motion in the Sloan Digital Sky Survey Data Release 14, we report the classification for 20 088 spectroscopically confirmed white dwarfs, plus 415 hot subdwarfs, and 311 cataclysmic variables. We obtain T eff , log g and mass for hydrogen atmosphere white dwarf stars (DAs), warm helium atmosphere white dwarfs (DBs), hot subdwarfs (sdBs and sdOs), and estimate photometric T eff for white dwarf stars with continuum spectra (DCs). We find 15 793 sdAs and 447 dCs between the white dwarf cooling sequence and the main sequence, especially below T eff 10 000 K; most are likely low-mass metal-poor main sequence stars, but some could be the result of interacting binary evolution.

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.

Stars born with masses below around 10 solar masses end their lives as white dwarf stars. Their atmospheres are dominated by the lightest elements because gravitational diffusion brings the lightest element to the surface. We report the discovery of a white dwarf with an atmosphere completely dominated by oxygen, SDSS J124043.01+671034.68. After oxygen, the next most abundant elements in its atmosphere are neon and magnesium, but these are lower by a factor of ≥25 by number. The fact that no hydrogen or helium are observed is surprising. Oxygen, neon, and magnesium are the products of carbon burning, which occurs in stars at the high-mass end of pre-white dwarf formation. This star, a possible oxygen-neon white dwarf, will provide a rare observational test of the evolutionary paths toward white dwarfs.