Photometric Monitoring of the Coldest Known Brown Dwarf With the Spitzer Space Telescope*

Abstract

Because WISE J085510.83−071442.5 (hereafter WISE 0855-0714) is the coldest known brown dwarf (∼ 250 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 µm 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 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 Skemer et al. (2016). We find that robust evidence of such clouds is not yet available.

Constraints on magnetospheric radio emission from Y dwarfs

“…In contrast, were unable to conclusively constrain the presence of clouds or non-equilibrium chemistry in its atmosphere when comparing photometry in six optical and near-IR bands to model predictions. Finally, Esplin et al (2016) reported variability at 3.6 μm and 4.5 μm with peak-to-peak amplitudes between 3-5 per cent and also found insufficient evidence for water ice clouds in the atmosphere. Periodicity in the observed variability was inconclusive, with periods ranging between 6.8-9.0 h at 3.6 μm and 5.3-9.3 h at 4.5 μm for two different epochs.…”

“…WISE 1405+55 was discovered and initially classified as a Y0p? dwarf by Cushing et al (2011), Esplin et al (2016) report the listed Lomb-Scargle periodogram peaks for 3.6 μm or 4.5 μm, respectively, for two different epochs. They also report that a double sinusoid model in which one period is twice that of the other returns 9.7 +0.9 −0.8 /10.8 +0.7 −0.7 for 3.6 μm or 4.5 μm, respectively, the first epoch and 14 +2 −2 /13.3 +0.5 −0.4 for the second epoch.…”

“…Indeed, Pineda et al (2017) indicate that several conditions are required to generate significant auroral emission, regardless of the underlying engine, including at least rapid rotation and strong magnetic field strengths. Esplin et al (2016) and Cushing et al (2016) have reported rotational periods derived from infrared variability at 8.5 h for WISE 0855−07, 5.3-9.3 h for WISE 1405+55, and 3.0-6.0 h for WISE 1738+27. In comparison, all pulsing radio brown dwarfs have reported rotational periods between 1.77 and 3.89 h (Pineda et al 2017;Kao et al 2018, and references therein).…”

“…To access the strongest constraints on fully convective dynamo models, pushing magnetic field measurements to Y dwarfs and eventually exoplanets, such as hot Jupiters, is critical. While previous searches for radio emission from exoplanets have been attempted, the work presented here is the first such attempt for Y dwarfs and is motivated by the success of our above described selection strategy and recent discoveries of variability at near-and/or midinfrared bands for three Y dwarfs, WISE J140518.39+553421.3, WISE J085510.83−071442.5, and WISEP J173835.52+273258.9 (Cushing et al 2016;Esplin et al 2016;Leggett et al 2016). These detections of variability have been quite reasonably attributed to variations in atmospheric temperature or opacity (weather), but it has been argued that similar phenomena can be driven by auroral currents for the ∼10 per cent of objects that exhibit radio pulsing (Hallinan et al 2015;Kao et al 2016;Pineda et al 2017).…”

WISEA J083011.95+283716.0: A Missing Link Planetary-mass Object

“…However, Skemer et al (2016) and Morley et al (2018) disputed the Luhman & Esplin (2014) result and validated water ice clouds as the interpretation for the absorption features in the 3.4-5.2 μm spectra. Finally, photometric variability at a 4% level was detected in WISEJ0855−0714 in the mid-infrared (Esplin et al 2016), suggesting a patchy atmosphere, yet the cloud composition remains unclear. WISEAJ0830+2837, along with CWISEPJ1935−1546 and CWISEP1446−2317, fill an important gap in the Y-dwarf sequence to continuously map atmospheric composition across low temperatures down to Jupiter.…”