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We perform an analysis of the single white dwarf and the double degenerate binary populations in the solar neighbourhood following a population synthesis approach to investigate the effects of unresolved double degenerates in the white dwarf luminosity function. We consider all unresolved synthetic binaries to be associated with fictitious effective temperatures and surface gravities that are obtained in the same way as if these objects were observed as single point sources. We evaluate the effects of unresolved double white dwarfs assuming that the synthetic samples are observed both by the magnitude-limited SDSS and the volume-limited Gaia surveys, the latter limited to a distance of no more than 100pc. We find that, for our standard model, the impact of unresolved double degenerates in the white dwarf luminosity function derived from the Gaia sample is nearly negligible. Unresolved double degenerates are hence expected to have no effect on the age of the Galactic disc, nor on the star formation history from this population. However, for the SDSS sample, the effect of unresolved double degenerates is significant at the brighter bins (Mbol<6.5 mag), with the fraction of such systems reaching ~40% of the total white dwarf population at Mbol=6 mag. This indicates unresolved double degenerates may influence the constraints on the star formation history derived from the SDSS white dwarf sample.
We analyze the effect of the sedimentation of $^{22}$Ne on the local white dwarf luminosity function by studying scenarios under different Galactic metallicity models. We make use of an up-to-date population synthesis code based on Monte Carlo techni
Extremely low-mass white dwarfs (ELM WDs) are helium WDs with a mass less than $sim$$0.3rm;M_odot$. Most ELM WDs are found in double degenerates (DDs) in the ELM Survey led by Brown and Kilic. These systems are supposed to be significant gravitationa
We present optical spectra of 144 white dwarfs detected in the Montreal-Cambridge-Tololo (MCT) colorimetric survey, including 120 DA, 12 DB, 4 DO, 1 DQ, and 7 DC stars. We also perform a model atmosphere analysis of all objects in our sample using th
We report on the white dwarf cooling sequence of the old globular cluster NGC 6752, which is chemically complex and hosts a blue horizontal branch. This is one of the last globular cluster white dwarf (WD) cooling sequences accessible to imaging by t
A sample of white dwarfs is selected from SDSS DR3 imaging data using their reduced proper motions, based on improved proper motions from SDSS plus USNO-B combined data. Numerous SDSS and followup spectra (Kilic et al. 2005) are used to quantify comp