A Nearby Old Halo White Dwarf Candidate from the Sloan Digital Sky Survey

TL;DR

This paper reports the discovery of a nearby, old halo white dwarf candidate with unique optical properties, providing insights into cool white dwarf atmospheres and galactic halo membership.

Contribution

It presents the identification and detailed analysis of a new halo white dwarf candidate with unusual colors and discusses implications for white dwarf atmospheric models and galactic population.

Findings

Object has high proper motion and halo-like velocities.

Spectral energy distribution fits a cool hydrogen atmosphere at ~3800 K.

Optical colors are redder than current models predict.

Abstract

We report the discovery of a nearby, old, halo white dwarf candidate from the Sloan Digital Sky Survey. SDSS J110217.48+411315.4 has a proper motion of 1.75 arcsec/year and redder optical colors than all other known featureless (type DC) white dwarfs. We present SDSS imaging and spectroscopy of this object, along with near-infrared photometry obtained at the United Kingdom Infra-Red Telescope. Fitting its photometry with up-to-date model atmospheres, we find that its overall spectral energy distribution is fit reasonably well with a pure hydrogen composition and T_eff~3800 K (assuming log g=8). That temperature and gravity would place this white dwarf at 35 pc from the Sun with a tangential velocity of 290 km/s and space velocities consistent with halo membership; furthermore, its combined main sequence and white dwarf cooling age would be ~11 Gyr. However, if this object is a massive white dwarf, it could be a younger object with a thick disk origin. Whatever its origin, the optical colors of this object are redder than predicted by any current pure hydrogen, pure helium or mixed hydrogen-helium atmospheric model, indicating that there remain problems in our understanding of the complicated physics of the dense atmospheres of cool white dwarfs.