On the Nature of Ultracool White Dwarfs: Not so Cool Afterall

Abstract

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

Accepted for publication in The Astrophysical Journal (33 pages, 21 figures)

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• The 40 pc sample of white dwarfs from Gaia
• $\text{Gaia}$ white dwarfs within 40 pc III: spectroscopic observations of new candidates in the southern hemisphere
• Spectral analysis of ultra-cool white dwarfs polluted by planetary debris
• The spectral evolution of white dwarfs: where do we stand?
• Carbon dredge-up required to explain the Gaia white dwarf colour-magnitude bifurcation
• Structure and evolution of ultra-massive white dwarfs in general relativity
• Carbon-oxygen ultra-massive white dwarfs in general relativity
• Mass measurements and 3D orbital geometry of PSR J1933$-$6211
• Revisiting astrophysical bounds on continuous spontaneous localization models
• White Dwarf Stars in the Big Data Era
• A new method to retrieve the star formation history from white dwarf luminosity functions -- an application to the Gaia catalogue of nearby stars