2008MNRAS.385L..23R

We present B_J, R_59F_ and I_N 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, a feature characteristic of the cool, high-density environments found in the atmospheres of UCWDs. SSSJ1556-0806 therefore joins a list of less than 10 UCWDs displaying extreme flux suppression. Synthetic model fitting suggests an effective temperature of <3000K, 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 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 infrared photometry.