The brightest pure-H ultracool white dwarf

TL;DR

This paper reports the discovery and detailed analysis of LSR J0745+2627, the brightest pure-H ultracool white dwarf, using multi-survey data and spectroscopy, confirming its properties and population membership.

Contribution

The paper presents the first identification and spectroscopic confirmation of the brightest pure-H ultracool white dwarf, expanding knowledge of cool white dwarf populations.

Findings

Identified LSR J0745+2627 as a pure-H ultracool white dwarf.

Confirmed its effective temperature as 3880±90 K.

Determined its likely thick-disk/halo origin.

Abstract

We report the identification of LSR J0745+2627 in the United Kingdom InfraRed Telescope Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS) as a cool white dwarf with kinematics and age compatible with the thick-disk/halo population. LSR J0745+2627 has a high proper motion (890 mas/yr) and a high reduced proper motion value in the J band (H_J=21.87). We show how the infrared-reduced proper motion diagram is useful for selecting a sample of cool white dwarfs with low contamination. LSR J0745+2627 is also detected in the Sloan Digital Sky Survey (SDSS) and the Wide-field Infrared Survey Explorer (WISE). We have spectroscopically confirmed this object as a cool white dwarf using X-Shooter on the Very Large Telescope. A detailed analysis of its spectral energy distribution reveals that its atmosphere is compatible with a pure-H composition model with an effective temperature of 3880+-90 K. This object is the brightest pure-H ultracool white dwarf (Teff<4000 K) ever identified. We have constrained the distance (24-45 pc), space velocities and age considering different surface gravities. The results obtained suggest that LSR J0745+2627 belongs to the thick-disk/halo population and is also one of the closest ultracool white dwarfs.