Photometric Monitoring of the Coldest Known Brown Dwarf with the Spitzer Space Telescope

TL;DR

This study used the Spitzer Space Telescope to monitor the coldest known brown dwarf, WISE 0855-0714, revealing variability that informs understanding of its atmospheric composition and cloud coverage in a temperature regime similar to exoplanets.

Contribution

First time detailed mid-infrared time-series photometry of WISE 0855-0714, providing insights into its atmospheric variability and cloud properties at very low temperatures.

Findings

Detected 3-5% variability in mid-IR bands.

Variability patterns are semi-periodic and irregular.

Small amplitude variability suggests minimal hemispheric cloud coverage.

Abstract

Because WISE J085510.83$-$071442.5 (hereafter WISE 0855-0714) is the coldest known brown dwarf ($\sim250$ K) and one of the Sun's closest neighbors (2.2 pc), it offers a unique opportunity for studying a planet-like atmosphere in an unexplored regime of temperature. To detect and characterize inhomogeneities in its atmosphere (e.g., patchy clouds, hot spots), we have performed time-series photometric monitoring of WISE 0855-0714 at 3.6 and 4.5 micron with the Spitzer Space Telescope during two 23~hr periods that were separated by several months. For both bands, we have detected variability with peak-to-peak amplitudes of 4-5% and 3-4% in the first and second epochs, respectively. The light curves are semi-periodic in the first epoch for both bands, but are more irregular in the second epoch. Models of patchy clouds have predicted a large increase in mid-IR variability amplitudes (for a given cloud covering fraction) with the appearance of water ice clouds at $T_{\rm eff}<$375 K, so if such clouds are responsible for the variability of WISE 0855-0714, then its small amplitudes of variability indicate a very small deviation in cloud coverage between hemispheres. Alternatively, the similarity in mid-IR variability amplitudes between WISE 0855-0714 and somewhat warmer T and Y dwarfs may suggest that they share a common origin for their variability (i.e., not water clouds). In addition to our variability data, we have examined other constraints on the presence of water ice clouds in the atmosphere of WISE 0855-0714, including the recent mid-IR spectrum from \citet{ske16}. We find that robust evidence of such clouds is not yet available.