Deep $z$-band observations of the coolest Y dwarf

Abstract

WISE J085510.83-071442.5 (hereafter, WISE 0855-07) is the coolest Y dwarf known to date and is located at a distance of 2.31$\pm 0.08$ pc, giving it the fourth largest parallax of any known star or brown dwarf system. We report deep $z$-band observations of WISE 0855-07 using FORS2 on UT1/VLT. We do not detect any counterpart to WISE 0855-07 in our $z$-band images and estimate a brightness upper limit of AB mag $>$ 24.8 ($F_{\nu }$ $<$ 0.45 $\mu$Jy) at 910 $\pm$ 65 nm with $3\sigma$-confidence. We combine our z-band upper limit with previous near- and mid-infrared photometry to place constraints on the atmospheric properties of WISE 0855-07 via comparison to models which implement water clouds in the atmospheres of $T_{eff}<300$ K substellar objects. We find that none of the available models that implement water clouds can completely reproduce the observed SED of WISE 0855-07. Every model significantly disagrees with the (3.6 $\mu$m / 4.5 $\mu$m) flux ratio and at least one other bandpass. Since methane is predicted to be the dominant absorber at 3-4 $\mu$m, these mismatches might point to an incorrect or incomplete treatment of methane in current models. We conclude that \mbox{(a) WISE0855-07} has $T_{eff}\sim 200-250$~K, (b) $<80\%$ of its surface is covered by clouds, and (c) deeper observations, and improved models of substellar evolution, atmospheres, clouds, and opacities will be necessary to better characterize this object.

arXiv comment13 pages, 3 figures; accepted to ApJ

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• Photometric Monitoring of the Coldest Known Brown Dwarf with the Spitzer Space Telescope
• A new take on the low-mass brown dwarf companions on wide-orbits in Upper-Scorpius
• Atmospheric Habitable Zones in Cool Y Dwarf Atmospheres
• The Collapse of the Wien Tail in the Coldest Brown Dwarf? Hubble Space Telescope Near-Infrared Photometry of WISE J085510.83-071442.5
• Near-infrared photometry of WISE J085510.74$-$071442.5
• A search for millimeter emission from the coldest and closest brown dwarf with ALMA
• Time-resolved JWST Retrieval Analysis of the Coldest Brown Dwarf: Temperature and Chemical Variations in WISE 0855-07