White dwarfs in the SuperCOSMOS Sky Survey: the thin disc, thick disc and spheroid luminosity functions

We present a magnitude and proper motion limited catalogue of ∼10 000 white dwarf candidates, obtained from the SuperCOSMOS Sky Survey by means of reduced proper motion selection. This catalogue extends to magnitudes R ∼ 19.75 and proper motions as low as μ∼ 0.05 arcsec yr −1 , and covers nearly three quarters of the sky. Photometric parallaxes provide distance estimates accurate to ∼50 per cent. This catalogue is used to measure the luminosity functions for disc and spheroid white dwarfs, using strict velocity cuts to isolate subsamples belonging to each population. Disc luminosity functions measured in this manner are really a conglomerate of thin and thick disc objects, due to the significant velocity overlap between these populations. We introduce a new statistical approach to the stellar luminosity function for nearby objects that successfully untangles the contributions from the different kinematic populations, without the need for stringent velocity cuts. This improves the statistical power by allowing all stars to contribute to the luminosity function, even at tangential velocities where the populations are indistinguishable. This method is particularly suited to white dwarfs, for which population discrimination by chemical tagging is not possible. We use this technique to obtain the first measurement of the thick disc white dwarf luminosity function, while also improving constraint on both the thin disc and spheroid luminosity functions. We find that the thin disc, thick disc and spheroid populations contribute to the local white dwarf density in roughly 79 per cent/16 per cent/5 per cent proportions.