Ultracool white dwarfs and the age of the Galactic disc

Abstract

We present parallax observations and a detailed model atmosphere analysis of 54 cool and ultracool ($T_{\mathrm{eff}}$ < 4000 K) white dwarfs (WDs) in the solar neighbourhood. For the first time, a large number of cool and ultracool WDs have distance and tangential velocities measurements available. Our targets have distances ranging from 21 pc to >100 pc, and include five stars within 30 pc. Contrary to expectations, all but two of them have tangential velocities smaller than 150 km s${}^{-1}$ thus suggesting Galactic disc membership. The oldest WDs in this sample have WD cooling ages of 10 Gyr, providing a firm lower limit to the age of the thick disc population. Many of our targets have uncharacteristically large radii, indicating that they are low mass WDs. It appears that we have detected the brighter population of cool and ultracool WDs near the Sun. The fainter population of ultracool CO-core WDs remain to be discovered in large numbers. The Large Synoptic Survey Telescope should find these elusive, more massive ultracool WDs in the solar neighbourhood.

16 pages, 11 figures, 4 tables, accepted for publication in MNRAS

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Cited by in corpus (9)

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• A New Generation of Cool White Dwarf Atmosphere Models. IV. Revisiting the Spectral Evolution of Cool White Dwarfs
• The Solar Neighborhood. XXXIX. Parallax Results from the CTIOPI and NOFS Programs: 50 New Members of the 25 Parsec White Dwarf Sample
• Random Forest identification of the thin disk, thick disk and halo Gaia-DR2 white dwarf population
• A Deep Proper Motion Catalog Within the Sloan Digital Sky Survey Footprint. II. The White Dwarf Luminosity Function
• Pressure Distortion of the H${}_2$-He Collision-Induced Absorption at the Photosphere of Cool White Dwarf Stars
• The ultracool helium-atmosphere white dwarf companion of PSR J0740+6620?
• The DECam Minute Cadence Survey I
• New Halo White Dwarf Candidates in the Sloan Digital Sky Survey