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

TL;DR

This study constrains the temperature of the coldest known brown dwarf, WISE 0855-0714, using near-infrared observations and model comparisons, confirming its status as the coldest brown dwarf with an estimated T_eff below 250 K.

Contribution

First near-infrared observational limit for WISE 0855-0714, refining its temperature estimate and confirming its status as the coldest brown dwarf.

Findings

WISE 0855-0714 is the reddest brown dwarf detected.

Effective temperature is constrained to below 250 K.

Non-detection in Y-band sets strong upper brightness limits.

Abstract

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 $\sim$2.2 pc away, and to improve its temperature estimate (T$_{\rm 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$_{\rm eff}<$500\,K theoretical models predict strong signal (or rather less molecular absorption) in this band. Results. WISE0855-0714 was not detected in our Y-band images, thus placing an upper limit on its brightness to Y>24.4 mag at 3-$\sigma$ level, leading to Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at other wavelengths, WISE0855$-$0714 is confirmed as the reddest BD detected, further supporting its status as the coldest known brown dwarf. We applied spectral energy distribution fitting with collections of models from two independent groups for extremely cool BD atmospheres leading to an effective temperature of T$_{\rm eff}<$250\,K,.