DIRECTLY IMAGED L-T TRANSITION EXOPLANETS IN THE MID-INFRARED^,

Abstract

Gas-giant planets emit a large fraction of their light in the mid-infrared\n($\\gtrsim$3$\\mu$m), where photometry and spectroscopy are critical to our\nunderstanding of the bulk properties of extrasolar planets. Of particular\nimportance are the L and M-band atmospheric windows (3-5$\\mu$m), which are the\nlongest wavelengths currently accessible to ground-based, high-contrast\nimagers. We present binocular LBT AO images of the HR 8799 planetary system in\nsix narrow-band filters from 3-4$\\mu$m, and a Magellan AO image of the 2M1207\nplanetary system in a broader 3.3$\\mu$m band. These systems encompass the five\nknown exoplanets with luminosities consistent with L$\\rightarrow$T transition\nbrown dwarfs. Our results show that the exoplanets are brighter and have\nshallower spectral slopes than equivalent temperature brown dwarfs in a\nwavelength range that contains the methane fundamental absorption feature\n(spanned by the narrowband filters and encompassed by the broader 3.3$\\mu$m\nfilter). For 2M1207 b, we find that thick clouds and non-equilibrium chemistry\ncaused by vertical mixing can explain the object's appearance. For the HR 8799\nplanets, we present new models that suggest the atmospheres must have patchy\nclouds, along with non-equilibrium chemistry. Together, the presence of a\nheterogeneous surface and vertical mixing presents a picture of dynamic\nplanetary atmospheres in which both horizontal and vertical motions influence\nthe chemical and condensate profiles.\n