Clouds and Chemistry in the Atmosphere of Extrasolar Planet HR8799b

Details Clouds and Chemistry in the Atmosphere of Extrasolar Planet HR8799b Clouds and Chemistry in the Atmosphere of Extrasolar Planet HR8799b

2011-03-20

arxiv.org/abs/1103.3895v1

Astronomy and Astrophysics

Astrophysics

Earth and Planetary Astrophysics

17 pages, accepted for publication in The Astrophysical Journal

Scientific paper

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 narrow-band 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, Teff = 1100K +- 100 and log(g) = 3.5 +-0.5 (for solar composition). Given the observed luminosity, log(Lobs/Lsun) ~ -5.1, these values correspond to a radius of 0.75 Rjup (+0.17, -0.12) and mass ~ 0.72 Mjup (+2.6,-0.6) -- strikingly inconsistent with interior/evolution models. Enhanced metallicity (up to ~ 10 times 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 (< 1000K) required by the evolution models.

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