A New Ultracool White Dwarf Discovered in the SuperCOSMOS Sky Survey

TL;DR

This paper reports the discovery and analysis of a new ultracool white dwarf with extreme flux suppression, potentially making it one of the coolest known white dwarfs, and discusses its spectral features and atmospheric properties.

Contribution

The paper introduces a newly discovered ultracool white dwarf with unique spectral features and provides initial atmospheric parameter estimates using model fitting and comparison with similar objects.

Findings

Shows strong flux suppression due to H2CIA in spectrum

Estimated effective temperature below 3000K

Identifies the object as one of the coolest white dwarfs known

Abstract

We present photographic B, R and I photometry, and optical and near-infrared spectroscopy, of a new ultracool white dwarf (UCWD) discovered in the SuperCOSMOS Sky Survey. The spectrum of SSSJ1556-0806 shows strong flux suppression due to the presence of collisionally induced absorption by molecular hydrogen (H2CIA), a feature characteristic of the cool, high density environments found in the atmospheres of ultracool white dwarfs. SSSJ1556-0806 therefore joins a list of <10 ultracool white dwarfs displaying extreme flux suppression. Synthetic model fitting suggests an effective temperature <3000K, which if true would make this one of the coolest white dwarfs currently known. We also exploit the similarity between the SEDs of SSSJ1556-0806 and the well-studied UCWD LHS 3250 to aid in the determination of the atmospheric parameters in a regime where models consistently fail to reproduce observations. SSSJ1556-0806 is relatively bright (R ~ 17.8), making it particularly amenable to follow up observations to obtain trigonometric parallax and IR photometry.