Orbital and spectral analysis of the benchmark brown dwarf HD 4747B ^★

¹ Observatoire astronomique de l’Université de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
e-mail: sebastien.peretti@unige.ch
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Max Planck Institute for Astronomy, Koenigstuhl 17 69117 Heidelberg, Germany
⁴ Laboratoire d’Astrophysique de Marseille, 38, rue Frédéric Joliot-Curie 13388 Marseille cedex 13, France
⁵ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC University of Paris 06, Université Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
⁶ Department of Physics, University of Oxford, Oxford, UK
⁷ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
⁸ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁹ Center for Space and Habitability, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
e-mail: kevin.heng@csh.unibe.ch
¹⁰ Université Cote d’Azur, OCA, CNRS, Lagrange, France
¹¹ Institute for Astronomy, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093 Zurich, Switzerland
¹² INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹³ Centre de Recherche Astrophysique de Lyon, CNRS, Université Lyon 1, 9 avenue Charles André, 69561 Saint Genis-Laval-Cedex, France
¹⁴ The University of Michigan, Ann Arbor, MI 48109, USA
¹⁵ NOVA Optical Infrared Instrumentation Group, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹⁶ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁷ Núcleo de Astonomía, Facultad de Ingeniería, Universidad Diego Portales, A. Ejercito 441, Santiago, Chile
¹⁸ Escuela de Ingeniería Industrial, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile

Received: 12 December 2017
Accepted: 13 May 2018

Abstract

Context. The study of high-contrast imaged brown dwarfs and exoplanets depends strongly on evolutionary models. To estimate the mass of a directly imaged substellar object, its extracted photometry or spectrum is used and adjusted with model spectra together with the estimated age of the system. These models still need to be properly tested and constrained. HD 4747B is a brown dwarf close to the H burning mass limit, orbiting a nearby (d = 19.25 ± 0.58 pc), solar-type star (G9V); it has been observed with the radial velocity method for over almost two decades. Its companion was also recently detected by direct imaging, allowing a complete study of this particular object.

Aims. We aim to fully characterize HD 4747B by combining a well-constrained dynamical mass and a study of its observed spectral features in order to test evolutionary models for substellar objects and to characterize its atmosphere.

Methods. We combined the radial velocity measurements of High Resolution Echelle Spectrometer (HIRES) and CORALIE taken over two decades and high-contrast imaging of several epochs from NACO, NIRC2, and SPHERE to obtain a dynamical mass. From the SPHERE data we obtained a low-resolution spectrum of the companion from Y to H band, and two narrow band-width photometric measurements in the K band. A study of the primary star also allowed us to constrain the age of the system and its distance.

Results. Thanks to the new SPHERE epoch and NACO archival data combined with previous imaging data and high-precision radial velocity measurements, we were able to derive a well-constrained orbit. The high eccentricity (e = 0.7362 ± 0.0025) of HD 4747B is confirmed, and the inclination and the semi-major axis are derived (i = 47.3 ± 1.6°, a = 10.01 ± 0.21 au). We derive a dynamical mass of m_B = 70.0 ± 1.6 M_Jup, which is higher than a previous study but in better agreement with the models. By comparing the object with known brown dwarfs spectra, we derive a spectral type of L9 and an effective temperature of 1350 ± 50 K. With a retrieval analysis we constrain the oxygen and carbon abundances and compare them with the values from the HR 8799 planets.