Temperature constraints on the coldest brown dwarf known: WISE 0855-0714

Context. Nearby isolated planetary mass objects are beginning to be discovered, but their individual properties are poorly constrained because their low surface temperatures and strong molecular self-absorption make them extremely faint. Aims. We aimed to detect the near-infrared emission of the coldest brown dwarf (BD) found so far, WISE0855-0714, located ~2.2 pc away, and to improve its temperature estimate (T_eff = 225-260 K) from a comparison with state-of-the-art models of BD atmospheres. Methods. We observed the field containing WISE0855-0714 with HAWK-I at the VLT in the Y band. For BDs with T_eff< 500 K theoretical models predict strong signal (or rather less molecular absorption) in this band. Results. WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y> 24.4 mag at 3σ level, leading to Y-[4.5] > 10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855-0714 is confirmed as the reddest BD detected, further supporting its status as the coldest known brown dwarf.