Ultracool white dwarfs and the age of the Galactic disc

TL;DR

This study analyzes 54 cool and ultracool white dwarfs near the Sun, providing new insights into their properties, velocities, and implications for the age of the Galactic disc, with many being low-mass and nearby.

Contribution

First comprehensive parallax and atmosphere analysis of a large sample of ultracool white dwarfs, revealing their velocities, masses, and age implications for the Galactic disc.

Findings

Most WDs have velocities indicating disc membership.

Oldest WDs suggest a minimum age of 10 Gyr for the thick disc.

Many WDs are low mass with larger radii than expected.

Abstract

We present parallax observations and a detailed model atmosphere analysis of 54 cool and ultracool ($T_{\rm eff}$ < 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.