HELIOS-RETRIEVAL: An Open-source, Nested Sampling Atmospheric Retrieval Code; Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation

Baptiste Lavie , João M. Mendonça , Christoph Mordasini , Matej Malik , Mickaël Bonnefoy (1) , Brice-Olivier Demory , Maria Oreshenko , Simon L. Grimm , David Ehrenreich , Kevin Heng

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

We present an open-source retrieval code named HELIOS-RETRIEVAL, designed to obtain chemical abundances and temperature-pressure profiles by inverting the measured spectra of exoplanetary atmospheres. In our forward model, we use an exact solution of the radiative transfer equation, in the pure absorption limit, which allows us to analytically integrate over all of the outgoing rays. Two chemistry models are considered: unconstrained chemistry and equilibrium chemistry (enforced via analytical formulae). The nested sampling algorithm allows us to formally implement Occam’s Razor based on a comparison of the Bayesian evidence between models. We perform a retrieval analysis on the measured spectra of the four HR 8799 directly imaged exoplanets. Chemical equilibrium is disfavored for HR 8799b and c. We find supersolar C/H and O/H values for the outer HR 8799b and c exoplanets, while the inner HR 8799d and e exoplanets have a range of C/H and O/H values. The C/O values range from being superstellar for HR 8799b to being consistent with stellar for HR 8799c and being substellar for HR 8799d and e. If these retrieved properties are representative of the bulk compositions of the exoplanets, then they are inconsistent with formation via gravitational instability (without late-time accretion) and consistent with a core accretion scenario in which late-time accretion of ices occurred differently for the inner and outer exoplanets. For HR 8799e, we find that spectroscopy in the K band is crucial for constraining C/O and C/H. HELIOS-RETRIEVAL is publicly available as part of the Exoclimes Simulation Platform (http://www.exoclime.org).

Mots clés: methods: statistical; planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: formation; planets and satellites: gaseous planets; planets and satellites: individual: HR 8799; Astrophysics - Earth and Planetary Astrophysics; Physics - Atmospheric and Oceanic Physics

Domaines: Planète et Univers [physics]

Baptiste Lavie, João M. Mendonça, Christoph Mordasini, Matej Malik, Mickaël Bonnefoy, et al.. HELIOS-RETRIEVAL: An Open-source, Nested Sampling Atmospheric Retrieval Code; Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation. The Astronomical Journal, 2017, 154, ⟨10.3847/1538-3881/aa7ed8⟩. ⟨insu-03692482⟩