Weather on Other Worlds. Ii. Survey Results: Spots Are Ubiquitous on L and T Dwarfs

Abstract

We present results from the Weather on Other Worlds Spitzer Exploration Science program to investigate photometric variability in L and T dwarfs, usually attributed to patchy clouds. We surveyed 44 L3-T8 dwarfs, spanning a range of J − K s colors and surface gravities. We find that 14/23 (61% +17% −20% , 95% confidence) of our single L3-L9.5 dwarfs are variable with peak-to-peak amplitudes between 0.2% and 1.5%, and 5/16 (31% +25% −17% ) of our single T0-T8 dwarfs are variable with amplitudes between 0.8% and 4.6%. After correcting for sensitivity, we find that 80% +20% −27% of L dwarfs vary by 0.2%, and 36% +26% −17% of T dwarfs vary by 0.4%. Given viewing geometry considerations, we conclude that photospheric heterogeneities causing >0.2% 3-5 μm flux variations are present on virtually all L dwarfs, and probably on most T dwarfs. A third of L dwarf variables show irregular light curves, indicating that L dwarfs may have multiple spots that evolve over a single rotation. Also, approximately a third of the periodicities are on timescales >10 hr, suggesting that slowly rotating brown dwarfs may be common. We observe an increase in the maximum amplitudes over the entire spectral type range, revealing a potential for greater temperature contrasts in T dwarfs than in L dwarfs. We find a tentative association (92% confidence) between low surface gravity and high-amplitude variability among L3-L5.5 dwarfs. Although we can not confirm whether lower gravity is also correlated with a higher incidence of variables, the result is promising for the characterization of directly imaged young extrasolar planets through variability.

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

“…The amplitudes that we have measured for WISE 0855-0714 (∼ 250 K) are only a few percent, which would require very small deviations in cloud coverage between hemispheres (∆h ∼ 0.01) if water clouds are responsible for the variability. Meanwhile, the mid-IR amplitudes for WISE 0855-0714 are similar to those observed for early Y dwarfs (∼ 400 K, Cushing et al 2016;Leggett et al 2016) and late T dwarfs (800-1000 K, Metchev et al 2015;Yang et al 2016), which should be too warm to harbor water clouds. If water clouds are producing the variability of WISE 0855-0714, then it must coincidentally have a value of ∆h that produces roughly the same amplitudes as the different variability mechanism that operates in those objects.…”

Maps of Evolving Cloud Structures in Luhman 16ab From HST Time-Resolved Spectroscopy

“…Radigan et al (2014) and Radigan (2014) found that -+ 39 14 16 % of brown dwarfs in the L/T transition are variable, and showed that high-amplitude variability (2%) is uncommon in the L/T transition (24 -+ 9 11 %). Metchev et al (2015) reached a similar conclusion showing that few L/T dwarfs have amplitudes larger than 1%-2%. The fact that both Luhman 16A and B present high-amplitude variability makes this brown dwarf binary a unique object.…”

A Physical Model-based Correction for Charge Traps in the Hubble Space Telescope’s Wide Field Camera 3 Near-IR Detector and Its Applications to Transiting Exoplanets and Brown Dwarfs

“…The survey conducted by Buenzli et al (2014) concluded that at least one-third of brown dwarfs with spectral types from mid-L to mid-T have observable rotational modulations in the near-IR band from 1.1 to 1.7 µm. Using Spitzer observations, Metchev et al (2015) also detected rotational modulations for 30% to 40% L3-L9.5 brown dwarfs in Spitzer 3.6 and 4.5 µm band. They both claimed that almost all brown dwarfs have heterogeneous cloud coverages, after taking into account the random distributions of the inclinations of rotation axis and limited time coverage and sensitivity of their observations.…”