On the Nature of Ultracool White Dwarfs: Not so Cool after All

Abstract A recent analysis of the 100 pc white dwarf sample in SDSS footprint demonstrated for first time existence a well-defined ultracool—or IR-faint—white sequence Hertzsprung–Russell diagram. Here we take advantage this discovery to enlarge IR-faint threefold. We expand our selection entire Pan-STARRS survey as well Montreal White Dwarf Database and identify 37 candidates with strong flux deficits optical. present follow-up Gemini optical spectroscopy 30 these systems confirm all them dwarfs. an additional set 33 objects based on their colors magnitudes. detailed model atmosphere 70 newly identified dwarfs together 35 previously known reported literature. discuss physics atmospheres show that key physical ingredient missing previous generation was high-density correction He ? free–free absorption coefficient. With new calculated purpose analysis, now obtain significantly higher effective temperatures larger stellar masses than T eff M values analyses, thus solving two-decade-old problem. In particular, group ultramassive Debye cooling phase parameters never measured before.