| Abstract | Using the integral field spectrograph OSIRIS, on the Keck II telescope, broad near-infrared H- and K-band spectra of the young exoplanet HR8799b have been obtained. In addition, six new narrowband photometric measurements have been taken across the H and K bands. These data are combined with previously published photometry for an analysis of the planet's atmospheric properties. Thick photospheric dust cloud opacity is invoked to explain the planet's red near-IR colors and relatively smooth near-IR spectrum. Strong water absorption is detected, indicating a hydrogen-rich atmosphere. Only weak CH(4) absorption is detected at K band, indicating efficient vertical mixing and a disequilibrium CO/CH(4) ratio at photospheric depths. The H-band spectrum has a distinct triangular shape consistent with low surface gravity. New giant planet atmosphere models are compared to these data with best-fitting bulk parameters, T(eff) = 1100 K +/- 100 and log(g) = 3.5 +/- 0.5 (for solar composition). Given the observed luminosity (log L(obs)/L(circle dot) similar to -5.1), these values correspond to a radius of 0.75 R(Jup-0.12)(+0.17) and a mass of similar to 0.72 M(Jup-0.6)(+2.6)-strikingly inconsistent with interior/evolution models. Enhanced metallicity (up to similar to 10x that of the Sun) along with thick clouds and non-equilibrium chemistry are likely required to reproduce the complete ensemble of spectroscopic and photometric data and the low effective temperatures (< 1000 K) required by the evolution models. |
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