Spectral and atmospheric characterization of 51 Eridani b using VLT/SPHERE^⋆,⋆⋆

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: samland@mpia.de
² International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD), 69117 Heidelberg, Germany
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ ONERA, The French Aerospace Lab BP72, 29 avenue de la Division Leclerc, 92322 Châtillon Cedex, France
⁵ Geneva Observatory, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
⁶ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ Institute for Astronomy, University of Edinburgh, Blackford Hill View, Edinburgh EH9 3HJ, UK
⁸ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Vitacura, Santiago 19, Chile
⁹ Department of Physics & Astronomy, College of Charleston, 66 George Street, Charleston, SC 29424, USA
¹⁰ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
¹¹ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
¹² Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
¹³ CRAL, UMR 5574, CNRS, Université Lyon 1, 9 avenue Charles André, 69561 Saint Genis Laval Cedex, France
¹⁴ Aix-Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13013 Marseille, France
¹⁵ Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
¹⁶ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁷ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile
¹⁸ NASA Ames Research Center, Moffett Field, CA 94035, USA
¹⁹ LESIA, Observatoire de Paris, PSL Research Univ., CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, UPMC Paris 6, Sorbonne Univ., 5 place Jules Janssen, 92195 Meudon Cedex, France
²⁰ School of Earth & Space Exploration, Arizona State University, Tempe, AZ 85287, USA
²¹ Ural Federal University, 620002 Yekaterinburg, Russia
²² Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange, 06304 Nice Cedex 4, France

Received: 21 September 2016
Accepted: 10 April 2017

Abstract

Context. 51 Eridani b is an exoplanet around a young (20 Myr) nearby (29.4 pc) F0-type star, which was recently discovered by direct imaging. It is one of the closest direct imaging planets in angular and physical separation (~0.5′′, ~13 au) and is well suited for spectroscopic analysis using integral field spectrographs.

Aims. We aim to refine the atmospheric properties of the known giant planet and to constrain the architecture of the system further by searching for additional companions.

Methods. We used the extreme adaptive optics instrument SPHERE at the Very Large Telescope (VLT) to obtain simultaneous dual-band imaging with IRDIS and integral field spectra with IFS, extending the spectral coverage of the planet to the complete Y- to H-band range and providing additional photometry in the K12-bands (2.11, 2.25 μm). The object is compared to other known cool and peculiar dwarfs. The posterior probability distributions for parameters of cloudy and clear atmospheric models are explored using MCMC. We verified our methods by determining atmospheric parameters for the two benchmark brown dwarfs Gl 570D and HD 3651B. We used archival VLT-NACO (L′) Sparse Aperture Masking data to probe the innermost region for additional companions.

Results. We present the first spectrophotometric measurements in the Y and K bands for the planet and revise its J-band flux to values 40% fainter than previous measurements. Cloudy models with uniform cloud coverage provide a good match to the data. We derive the temperature, radius, surface gravity, metallicity, and cloud sedimentation parameter f_sed. We find that the atmosphere is highly super-solar ([Fe/H] = 1.0 ± 0.1 dex), and the low ${\mathit{f}}_{\mathrm{sed}}\mathrm{=}\mathrm{1.2}{\mathrm{6}}_{-0.29}^{\mathrm{+}\mathrm{0.36}}$ value is indicative of a vertically extended, optically thick cloud cover with small sized particles. The model radius and surface gravity estimates suggest higher planetary masses of ${\mathit{M}}_{\mathrm{gravity}}\mathrm{=}\mathrm{9.}{\mathrm{1}}_{-3.3}^{\mathrm{+}\mathrm{4.9}}\hspace{0.17em}{\mathit{M}}_{\mathrm{J}}$. The evolutionary model only provides a lower mass limit of > 2 M_J (for pure hot-start). The cold-start model cannot explain the luminosity of the planet. The SPHERE and NACO/SAM detection limits probe the 51 Eri system at solar system scales and exclude brown-dwarf companions more massive than 20 M_J beyond separations of ~2.5 au and giant planets more massive than 2 M_J beyond 9 au.