CLOUDS AND CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HR8799b

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 CH₄ absorption is detected at K band, indicating efficient vertical mixing and a disequilibrium CO/CH₄ 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 _☉ ~ – 5.1), these values correspond to a radius of 0.75 R _Jup ^+0.17 _{– 0.12} and a mass of ~0.72 M _Jup ^+2.6 _{– 0.6}—strikingly inconsistent with interior/evolution models. Enhanced metallicity (up to ~10× 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.