A new ultracool white dwarf discovered in the super COSMOS sky survey

Abstract

We present BJ, R59F and IN photometry, and optical and near-infrared spectroscopy, of a new ultracool white dwarf (UCWD) discovered in the SuperCOSMOS Sky Survey. The spectrum of SSS J1556-0806 shows strong flux suppression due to the presence of collisionally induced absorption by molecular hydrogen, a feature characteristic of the cool, high-density environments found in the atmospheres of UCWDs. SSS J1556-0806 therefore joins a list of less than 10 UCWDs displaying extreme flux suppression. Synthetic model fitting suggests an effective temperature of <3000 K, which if true would make this one of the coolest white dwarf currently known. We also exploit the similarity between the spectral energy distributions of SSS J1556-0806 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. SSS J1556-0806 is relatively bright (R ~ 17.8), making it particularly amenable to follow up observations to obtain trigonometric parallax and infrared photometry. © 2008 The Authors. Journal compilation © 2008 RAS.