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

Lavie, Baptiste; Mordasini, Christoph; Malik, Matej; Bonnefoy, Mickaël; Demory, Brice-Olivier; Oreshenko, Maria; Grimm, Simon L.; Ehrenreich, David; Heng, Kevin; Mendonca, Joao (2017). HELIOS–RETRIEVAL: An Open-source, Nested Sampling Atmospheric Retrieval Code, Application to the HR 8799 Exoplanets and Inferred Constraints for Planet Formation. Astronomical journal, 154(3), p. 91. University of Chicago Press 10.3847/1538-3881/aa7ed8

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We present an open-source retrieval code named HELIOS-Retrieval (hereafter HELIOS-R), designed to obtain chemical abundances and temperature-pressure profiles from inverting the measured spectra of exoplanetary atmospheres. In the current implementation, we use an exact solution of the radiative transfer equation, in the pure absorption limit, in our forward model, which allows us to analytically integrate over all of the outgoing rays (instead of performing Gaussian quadrature). Two chemistry models are considered: unconstrained chemistry (where the mixing ratios are treated as free parameters) and equilibrium chemistry (enforced via analytical formulae, where only the elemental abundances are free parameters). 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 HR 8799b, c, d and e directly imaged exoplanets. Chemical equilibrium is disfavored by the Bayesian evidence for HR 8799b, c and d. We find supersolar C/O, C/H and O/H values for the outer HR 8799b and c exoplanets, while the inner HR 8799d and e exoplanets have substellar C/O, substellar C/H and superstellar O/H values. 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-R is publicly available as part of the Exoclimes Simulation Platform (ESP; www.exoclime.org).

Item Type:

Journal Article (Original Article)

Division/Institute:

10 Strategic Research Centers > Center for Space and Habitability (CSH)
08 Faculty of Science > Physics Institute
08 Faculty of Science > Physics Institute > NCCR PlanetS
08 Faculty of Science > Physics Institute > Space Research and Planetary Sciences

UniBE Contributor:

Lavie, Baptiste; Mordasini, Christoph; Malik, Matej; Demory, Brice-Olivier; Oreshenko, Maria; Grimm, Simon L. and Heng, Kevin

Subjects:

500 Science
500 Science > 520 Astronomy
500 Science > 530 Physics
600 Technology > 620 Engineering

ISSN:

0004-6256

Publisher:

University of Chicago Press

Language:

English

Submitter:

Janine Jungo

Date Deposited:

18 Apr 2018 16:20

Last Modified:

18 Apr 2018 16:29

Publisher DOI:

10.3847/1538-3881/aa7ed8

BORIS DOI:

10.7892/boris.112678

URI:

https://boris.unibe.ch/id/eprint/112678

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