Near-Infrared Spectroscopy of the Extrasolar Planet HR 8799 b

TL;DR

This study presents high-contrast near-infrared spectroscopy of exoplanet HR 8799 b, revealing its atypical spectral features, atmospheric properties, and temperature discrepancies with models, suggesting it is a unique, dusty, low-luminosity object.

Contribution

First detailed near-infrared spectral analysis of HR 8799 b, highlighting its unusual atmospheric characteristics and challenging existing models of exoplanet atmospheres.

Findings

HR 8799 b shows little methane absorption, unlike similar T dwarfs.

Best-fit models suggest effective temperatures of 1300-1700 K, lower than evolutionary estimates.

HR 8799 b's properties imply a gravity-dependent L/T transition and a unique atmospheric composition.

Abstract

[Abridged] We present 2.12-2.23 um high contrast integral field spectroscopy of the extrasolar planet HR 8799 b. Our observations were obtained with OSIRIS on the Keck II telescope and sample the 2.2 um CH4 feature, which is useful for spectral classification and as a temperature diagnostic for ultracool objects. The spectrum of HR 8799 b is relatively featureless, with little or no methane absorption, and does not exhibit the strong CH4 seen in T dwarfs of similar absolute magnitudes. Overall, we find that HR 8799 b has a spectral type consistent with L5-T2, although its SED is atypical compared to most field objects. We fit the 2.2 um spectrum and the infrared SED using the Hubeny & Burrows, Burrows et al., and Ames-Dusty model atmosphere grids, which incorporate nonequilibrium chemistry, non-solar metallicities, and clear and cloudy variants. No models agree with all of the data, but those with intermediate clouds produce significantly better fits. The largest discrepancy occurs in the J-band, which is highly suppressed in HR 8799 b. The best-fitting effective temperatures range from 1300-1700 K with radii between ~0.3-0.5 RJup. These values are inconsistent with evolutionary model-derived values of 800-900 K and 1.1-1.3 RJup based on the luminosity of HR 8799 b and the age of HR 8799, a discrepancy that probably results from imperfect atmospheric models or the limited range of physical parameters covered by the models. The low temperature inferred from evolutionary models indicates that HR 8799 b is ~400 K cooler than field L/T transition objects, providing further evidence that the L/T transition is gravity-dependent. With an unusually dusty photosphere, an exceptionally low luminosity for its spectral type, and hints of extreme secondary physical parameters, HR 8799 b appears to be unlike any class of field brown dwarf currently known.