Friday, July 5, 2013

1307.1404 (Jae-Min Lee et al.)

Atmospheric Retrieval Analysis of the Directly Imaged Exoplanet HR 8799b    [PDF]

Jae-Min Lee, Kevin Heng, Patrick G. J. Irwin
Directly-imaged exoplanets are unexplored laboratories for the application of the spectral and temperature retrieval method, where the chemistry and composition of their atmospheres are inferred from inverse modeling of the available data. As a pilot study, we focus on the extrasolar gas giant HR 8799b for which more than 50 data points are available. We upgrade our non-linear optimal estimation retrieval method to include a phenomenological model of clouds that requires the cloud optical depth and monodisperse particle size to be specified. Previous studies have focused on forward models with assumed values of the exoplanetary properties; there is no consensus on the best-fit values of the radius, mass, surface gravity and effective temperature of HR 8799b. We show that cloudfree models produce reasonable fits to the data if the atmosphere is of super-solar metallicity and non-solar elemental abundances. Intermediately cloudy models with moderate values of the cloud optical depth and micron-sized particles provide a marginally better fit to the data and require a lower mean molecular weight. There is a mismatch between the photospheric and model radii, which we term the "radius ratio problem" and has been encountered in previous studies. By virtue of using an inverse model, we confirm that the radius ratio problem is not due to inadequacies with the evolutionary models. We report our best-fit values for the radius, mass, surface gravity and effective temperature of HR 8799b. The mean molecular weight is about 3.8, while the carbon-to-oxygen ratio is about unity due to the prevalence of carbon monoxide. Our study emphasizes the need for robust claims about the nature of an exoplanetary atmosphere to be based on analyses involving both photometry and spectroscopy and inferred from beyond a few photometric data points, such as are typically reported for hot Jupiters. [Abridged]
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