We have extended our detailed survey of the local white dwarf population from 20 to 25 pc, effectively doubling the sample volume, which now includes 232 stars. In the process, new stars within 20 pc have been added, a more uniform set of distance estimates as well as improved spectral and binary classifications are available. The present 25 pc sample is estimated to be about 68 per cent complete (the corresponding 20 pc sample is now 86 per cent complete). The space density of white dwarfs is unchanged at 4.8 ± 0.5 × 10 −3 pc −3 . This new study includes a white dwarf mass distribution and luminosity function based on the 232 stars in the 25 pc sample. We find a significant excess of single stars over systems containing one or more companions (74 per cent versus 26 per cent). This suggests mechanisms that result in the loss of companions during binary system evolution. In addition, this updated sample exhibits a pronounced deficiency of nearby 'Sirius-like' systems. 11 such systems were found within the 20 pc volume versus only one additional system found in the volume between 20 and 25 pc. An estimate of white dwarf birth rates during the last ∼8 Gyr is derived from individual remnant cooling ages. A discussion of likely ways new members of the local sample may be found is provided.Key words: binaries: close -white dwarfs.
I N T RO D U C T I O NThere is considerable interest in establishing a complete census of the population of nearby stars, particularly for those of the lowest luminosity, such as white dwarfs (WDs), late M-stars and sub-stellar L and T dwarfs. For the WD stars, previous publications (Holberg et al. 2002, 2008a, hereafter LWD02 and LWD08, respectively) have discussed the sample extending to 20 pc from the Sun. Since 2008, the 20 pc sample has grown by 10 members and distance estimates have been improved for many other WDs in this range. In addition, Giammichele, Bergeron & Dufour (2012 -hereafter GBD) have conducted a thorough spectral analysis of most WDs within the existing 20 pc sample. These developments provide a firm basis for a more homogeneous and unbiased determination of spectral types and stellar parameters. Thus, an improved knowledge base now exists with which to characterize the 20 pc local sample, since its completeness now approaches 86 per cent. In this paper, our previous 20 pc (LS20) sample is updated and extended out to a distance of 25 pc (LS25). Our formal sample limit is actually 25.2 pc, since at this distance, the added volume is exactly twice that of the original LS20 sample, which facilitates direct comparisons between the two subsamples. For example, the higher completeness E-mail: holberg@argus.lpl.arizona.edu factor of LS20 permits extrapolation of population properties to the less complete LS25.The LS25 is a volume-limited sample in which distance is the fundamental criterion for inclusion. Such a sample has several properties that are not easily matched by magnitude-limited samples that include many more WDs. For example, LS25 is an all-sky survey th...