Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: molliere@mpia.de
² Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
³ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
⁴ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cit, 5 place Jules Janssen, 92195 Meudon, France
⁵ Max Planck Institute for extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
⁶ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁹ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁰ CENTRA, Centro de Astrofísica e Gravitação, Instituto Superior Técnico, Avenida Rovisco Pais 1, 1049 Lisboa, Portugal
¹¹ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹² Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹³ Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
¹⁴ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
¹⁵ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁶ Space Telescope Science Institute, Baltimore, MD 21218, USA
¹⁷ Institute of Astronomy, Russian Academy of Sciences, 48 Pyatnitskaya St., Moscow, 119017, Russia
¹⁸ Department of Chemistry, Ludwig-Maximilians-Universität, Butenandtstraße 5-13, 81377 Munich, Germany
¹⁹ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
²⁰ Institut für Astronomie und Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
²¹ University of Exeter, Physics Building, Stocker Road, Exeter, EX4 4QL, UK
²² School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
²³ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

Received: 1 May 2020
Accepted: 16 June 2020

Abstract

Context. Clouds are ubiquitous in exoplanet atmospheres and they represent a challenge for the model interpretation of their spectra. When generating a large number of model spectra, complex cloud models often prove too costly numerically, whereas more efficient models may be overly simplified.

Aims. We aim to constrain the atmospheric properties of the directly imaged planet HR 8799e with a free retrieval approach.

Methods. We used our radiative transfer code petitRADTRANS for generating the spectra, which we coupled to the PyMultiNest tool. We added the effect of multiple scattering which is important for treating clouds. Two cloud model parameterizations are tested: the first incorporates the mixing and settling of condensates, the second simply parameterizes the functional form of the opacity.

Results. In mock retrievals, using an inadequate cloud model may result in atmospheres that are more isothermal and less cloudy than the input. Applying our framework on observations of HR 8799e made with the GPI, SPHERE, and GRAVITY, we find a cloudy atmosphere governed by disequilibrium chemistry, confirming previous analyses. We retrieve that C/O = 0.60_−0.08^+0.07. Other models have not yet produced a well constrained C/O value for this planet. The retrieved C/O values of both cloud models are consistent, while leading to different atmospheric structures: either cloudy or more isothermal and less cloudy. Fitting the observations with the self-consistent Exo-REM model leads to comparable results, without constraining C/O.

Conclusions. With data from the most sensitive instruments, retrieval analyses of directly imaged planets are possible. The inferred C/O ratio of HR 8799e is independent of the cloud model and thus appears to be a robust. This C/O is consistent with stellar, which could indicate that the HR 8799e formed outside the CO₂ or CO iceline. As it is the innermost planet of the system, this constraint could apply to all HR 8799 planets.