Letter to the Editor

A dusty benchmark brown dwarf near the ice line of HD 72946^⋆

¹ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
e-mail: almaire@uliege.be
² Department of Physics, University of Oxford, Oxford, UK
³ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ INAF Catania Astrophysical Observatory, Via S. Sofia 78, 95123 Catania, Italy
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Instituto de Física y Astronomía, Facultad de Ciencias, Univ. de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
⁷ Núcleo Milenio Formación Planetaria – NPF, Univ. de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Valparaíso, Chile
⁸ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
⁹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁰ Geneva Observatory, University of Geneva, Chemin des Maillettes 51, 1290 Versoix, Switzerland
¹¹ Unidad Mixta Internacional Franco-Chilena de Astronomía CNRS/INSU UMI 3386 and Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹² CRAL, UMR 5574, CNRS/ENS-Lyon/Université Lyon 1, 9 Av. Ch. André, 69561 Saint-Genis-Laval, France

Received: 18 November 2019
Accepted: 4 December 2019

Abstract

Context. HD 72946 is a bright and nearby solar-type star hosting a low-mass companion at long period (P ∼ 16 yr) detected with the radial velocity (RV) method. The companion has a minimum mass of 60.4 ± 2.2 M_J and might be a brown dwarf. Its expected semi-major axis of ∼243 mas makes it a suitable target for further characterization with high-contrast imaging, in particular to measure its inclination, mass, and spectrum and thus definitely establish its substellar nature.

Aims. We aim to further characterize the orbit, atmosphere, and physical nature of HD 72946B.

Methods. We present high-contrast imaging data in the near-infrared with the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument. We also use proper motion measurements of the star from HIPPARCOS and Gaia.

Results. The SPHERE data reveal a point source with a contrast of ∼9 mag at a projected separation of ∼235 mas. No other point sources are detected in the field of view. By jointly fitting the RV, imaging, and proper motion data, we constrain all the orbital parameters of HD 72946B and assess a dynamical mass of 72.4 ± 1.6 M_J and a semi-major axis of 6.456.45^+0.08_−0.07 au. Empirical comparison of its SPHERE spectrum to template dwarfs indicates a spectral type of L5.0 ± 1.5. The J–H3 color is close to the expectations of the DUSTY models and suggests a cloudy atmosphere. Comparison with atmospheric models of the spectrophotometry suggests an effective temperature of ∼1700 K. The bolometric luminosity (log(L/L_⊙) = −4.11 ± 0.10 dex) and dynamical mass of HD 72946B are more compatible with evolutionary models for an age range of ∼0.9−3 Gyr. The formation mechanism of the companion is currently unclear as the object appears slightly away from the bulk of model predictions. HD 72946B is currently the closest benchmark brown dwarf companion to a solar-type star with imaging, RV, and proper motion measurements.