An ultra-cool white dwarf serendipitously found with COMBO-17

Abstract

We report the discovery of an ultra-cool white dwarf in the COMBO-17 survey. So far, only seven objects have been discovered in this rare category of white dwarfs, which are characterized by strong flux depression in the far-red and near-infrared part of the spectrum, presumably due to collisionally induced absorption (CIA). The new object COMBO-17 J114356.08-0144032 has very similar colours to LHS 3250, which was the first of its kind to be recognized. However, at R=21.5 it is the faintest and possibly most distant such object discovered to date. It is the only such object in COMBO-17 at R<23; due to the small sky coverage of 0.78 square degree this chance discovery can not provide any constraints on the abundance of faint ultra-cool white dwarfs. We speculate on the basis of the proper motion that this new object is probably a member of the Galactic (thin) disk.