Photometric Identification of Cool White Dwarfs

We investigate the use of a narrow-band DDO51 filter for photometric identification of cool white dwarfs. We report photometric observations of 30 known cool white dwarfs with temperatures ranging from 10,000 K down to very cool temperatures (<3500 K). Follow-up spectroscopic observations of a sample of objects selected using this filter and our photometric observations show that DDO51 filter photometry can help select cool white dwarf candidates for follow-up multi–object spectroscopy by rejecting 65% of main sequence stars with the same broad–band colors as the cool white dwarfs. This technique is not selective enough to efficiently feed single–object spectrographs. We present the white dwarf cooling sequence using this filter. Our observations show that very cool white dwarfs form a sequence in the r-DDO vs. r-z color–color diagram and demonstrate that significant improvements are needed in white dwarf model atmospheres.