Letter to the Editor

Direct discovery of the inner exoplanet in the HD 206893 system

Evidence for deuterium burning in a planetary-mass companion

¹ University of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, UK
e-mail: shinkley@gmail.com
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
³ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁴ Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
⁵ Institüt für Astronomie und Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
⁶ Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁹ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
¹⁰ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹¹ Space Telescope Science Institute, Baltimore, MD 21218, USA
¹² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹³ Center for Astrophysics and Planetary Science, Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
¹⁴ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
¹⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁶ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁷ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
¹⁸ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹⁹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
²⁰ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
²¹ School of Physics and Astronomy, Monash University, Clayton, Melbourne, 3800 VIC, Australia
²² Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
²³ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
²⁴ Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
²⁵ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
²⁶ Astronomy Department, University of Michigan, Ann Arbor, MI 48109, USA
²⁷ European Southern Observatory, Casilla, 19001 Santiago 19, Chile
²⁸ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²⁹ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
³⁰ Department of Astronomy, Smith College, Northampton, MA 01063, USA
³¹ Pixyl S.A., 5 Av. du Grand Sablon, 38700 La Tronche, France
³² Department of Physics, University of Oxford, Oxford, UK
³³ Department of Physics and McGill Space Institute, McGill University, 3600 Rue University, Montreal, QC H3A 2T8, Canada
³⁴ Institut de Recherche sur les Exoplanètes (iREx), Université de Montréal, C.P. 6128 Succ. Centre-ville, Montreal, QC H3C 3J7, Canada
³⁵ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan
³⁶ Advanced Concepts Team, European Space Agency, TEC-SF, ESTEC, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

Received: 9 August 2022
Accepted: 25 October 2022

Abstract

Aims. HD 206893 is a nearby debris disk star that hosts a previously identified brown dwarf companion with an orbital separation of ∼10 au. Long-term precise radial velocity (RV) monitoring, as well as anomalies in the system proper motion, has suggested the presence of an additional, inner companion in the system.

Methods. Using information from ongoing precision RV measurements with the HARPS spectrograph, as well as Gaia host star astrometry, we have undertaken a multi-epoch search for the purported additional planet using the VLTI/GRAVITY instrument.

Results. We report a high-significance detection over three epochs of the companion HD 206893c, which shows clear evidence for Keplerian orbital motion. Our astrometry with ∼50−100 μarcsec precision afforded by GRAVITY allows us to derive a dynamical mass of 12.7${}_{-1.0}^{+1.2}$M_Jup and an orbital separation of 3.53${}_{-0.06}^{+0.08}$ au for HD 206893c. Our fits to the orbits of both companions in the system use both Gaia astrometry and RVs to also provide a precise dynamical estimate of the previously uncertain mass of the B component, and therefore allow us to derive an age of 155 ± 15 Myr for the system. We find that theoretical atmospheric and evolutionary models that incorporate deuterium burning for HD 206893c, parameterized by cloudy atmosphere models as well as a “hybrid sequence” (encompassing a transition from cloudy to cloud-free), provide a good simultaneous fit to the luminosity of both HD 206893B and c. Thus, accounting for both deuterium burning and clouds is crucial to understanding the luminosity evolution of HD 206893c.

Conclusions. In addition to using long-term RV information, this effort is an early example of a direct imaging discovery of a bona fide exoplanet that was guided in part by Gaia astrometry. Utilizing Gaia astrometry is expected to be one of the primary techniques going forward for identifying and characterizing additional directly imaged planets. In addition, HD 206893c is an example of an object narrowly straddling the deuterium-burning limit but unambiguously undergoing deuterium burning. Additional discoveries like this may therefore help clarify the discrimination between a brown dwarf and an extrasolar planet. Lastly, this discovery is another example of the power of optical interferometry to directly detect and characterize extrasolar planets where they form, at ice-line orbital separations of 2−4 au.