Name/Affiliation: Abhijith Rajan (Arizona State University)
Characterizing the Coolest Atmospheres: Exoplanets to Brown Dwarfs
We present the results of two complementary Brown dwarf Atmosphere Monitoring (BAM) programs characterizing planetary temperature sub-stellar L, T, and Y-dwarfs. The BAM-I project uses the SofI camera on the 3.5 m NTT to conduct an extensive Js-band monitoring survey of an unbiased sample of 69 brown dwarfs spanning the L0 and T8 spectral range. A total of 14 brown dwarfs were identified as variables with amplitudes ranging from 1.7% to 10.8% over the observed duration. Approximately half of the variables show sinusoidal amplitude variations similar to 2M2139, and the remainder shows aperiodic variations similar to SIMP0136. The survey was designed to test the hypothesis that the L/T transition has a higher degree of variability due to the presence of patchy clouds. The measured variability frequency for the BAM-I transition region variables is indistinct from that of the non-transition region brown dwarfs. In the BAM-II project we conducted a pilot study monitoring an initial sample of five ultracool T6.5 to Y0 brown dwarfs for infrared photometric variability using the SWIRC camera on the 6.5m MMT. T/Y transition objects with temperatures ranging from 500 – 900 K are expected to have salt and sulfide clouds form and breakup. One of the targets in the survey shows large amplitude peak-to-trough variations as high as 23% in our data, making it the highest amplitude variable brown dwarf detected outside the L/T transition region. With an effective temperature of 600 K, it is the coldest variable brown dwarf detected to-date. We are currently following-up all our candidate variables with multi-wavelength monitoring using a variety of ground-based telescopes. These brown dwarf variables will provide an invaluable dataset that will serve as a benchmark comparison to directly imaged planets and intensely irradiated Hot Jupiters and to synthetic atmospheric models incorporating different physical processes.