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

Stanimir A. Metchev
Aren Heinze
Daniel Apai
Davin Flateau
Jacqueline Radigan
Adam Burgasser
Mark S. Marley
Etienne Artigau
Peter Plavchan
Bertrand Goldman

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 - Ks colors and surface gravities. We find that 14/23 ($61\%_{-20\%}^{+17\%}$, 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 ($31\%_{-17\%}^{+25\%}$) of our single T0-T8 dwarfs are variable with amplitudes between 0.8% and 4.6%. After correcting for sensitivity, we find that $80\%_{-27\%}^{+20\%}$ of L dwarfs vary by ≥0.2%, and $36\%_{-17\%}^{+26\%}$ 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.