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\nScience program to investigate photometric variability in L and T dwarfs,\nusually attributed to patchy clouds. We surveyed 44 L3-T8 dwarfs, spanning a\nrange of $J-K_s$ colors and surface gravities. We find that 14/23 (61%; 95%\nconfidence interval: 41%-78%) of our single L3-L9.5 dwarfs are variable with\npeak-to-peak amplitudes between 0.2% and 1.5%, and 5/16 (31%; 95% confidence\ninterval: 14%-56%) of our single T0-T8 dwarfs are variable with amplitudes\nbetween 0.8% and 4.6%. After correcting for sensitivity, we find that 80% (95%\nconfidence interval: 53%-100%) of L dwarfs vary by >0.2%, and 36% (95%\nconfidence interval: 19%-52%) of T dwarfs vary by >0.4%. Given viewing geometry\nconsiderations, we conclude that photospheric heterogeneities causing >0.2%\n3-5-micron flux variations are present on virtually all L dwarfs, and probably\non most T dwarfs. A third of L dwarf variables show irregular light curves,\nindicating that L dwarfs may have multiple spots that evolve over a single\nrotation. Also, approximately a third of the periodicities are on time scales\n>10 h, suggesting that slowly-rotating brown dwarfs may be common. We observe\nan increase in the maximum amplitudes over the entire spectral type range,\nrevealing a potential for greater temperature contrasts in T dwarfs than in L\ndwarfs. We find a tentative association (92% confidence) between low surface\ngravity and high-amplitude variability among L3-L5.5 dwarfs. Although we can\nnot confirm whether lower gravity is also correlated with a higher incidence of\nvariables, the result is promising for the characterization of directly imaged\nyoung extrasolar planets through variability.\n