GRAVITY K-band spectroscopy of HD 206893 B^★

Brown dwarf or exoplanet

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
e-mail: j.kammerer.bw@googlemail.com
² Research School of Astronomy & Astrophysics, Australian National University, Canberra, ACT 2611, Australia
³ LESIA, Observatoire de Paris, PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Janssen, 92195 Meudon, France
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁸ Universidade de Lisboa - Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁹ CENTRA - Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹⁰ Max Planck Institute for extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
¹¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹³ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Melbourne, Australia
¹⁴ JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA
¹⁵ Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
¹⁶ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁷ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁸ Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
¹⁹ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
²⁰ Astronomy Department, University of Michigan, Ann Arbor, MI 48109, USA
²¹ Space Telescope Science Institute, Baltimore, MD 21218, USA
²² European Southern Observatory, Casilla 19001, Santiago 19, Chile
²³ University of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, UK
²⁴ STAR Institute/Université de Liège, Belgium
²⁵ Center for Astrophysics and Planetary Science, Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
²⁶ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA
²⁷ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
²⁸ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Sect. 4, Roosevelt Rd., Taipei 10617, Taiwan

Received: 8 March 2021
Accepted: 2 June 2021

Abstract

Context. Near-infrared interferometry has become a powerful tool for studying the orbital and atmospheric parameters of substellar companions.

Aims. We aim to reveal the nature of the reddest known substellar companion HD 206893 B by studying its near-infrared colors and spectral morphology and by investigating its orbital motion.

Methods. We fit atmospheric models for giant planets and brown dwarfs and perform spectral retrievals with petitRADTRANS and ATMO on the observed GRAVITY, SPHERE, and GPI spectra of HD 206893 B. To recover its unusual spectral features, first and foremost its extremely red near-infrared color, we include additional extinction by high-altitude dust clouds made of enstatite grains in the atmospheric model fits. However, forsterite, corundum, and iron grains predict similar extinction curves for the grain sizes considered here. We also infer the orbital parameters of HD 206893 B by combining the ~100 μas precision astrometry from GRAVITY with data from the literature and constrain the mass and position of HD 206893 C based on the Gaia proper motion anomaly of the system.

Results. The extremely red color and the very shallow 1.4 μm water absorption feature of HD 206893 B can be fit well with the adapted atmospheric models and spectral retrievals. By comparison with AMES-Cond evolutionary tracks, we find that only some atmosphericmodels predict physically plausible objects. Altogether, our analysis suggests an age of ~ 3–300 Myr and a mass of ~ 5–30 M_Jup for HD 206893 B, which is consistent with previous estimates but extends the parameter space to younger and lower-mass objects. The GRAVITY astrometry points to an eccentric orbit (e = 0.29_−0.11^+0.06) with a mutual inclination of <34.4 deg with respectto the debris disk of the system.

Conclusions. While HD 206893 B could in principle be a planetary-mass companion, this possibility hinges on the unknown influence of the inner companion on the mass estimate of 10₋₄⁺⁵ M_Jup from radial velocity and Gaia as well as a relatively small but significant Argus moving group membership probability of ~ 61%. However, we find that if the mass of HD 206893 B is <30 M_Jup, then the inner companion HD 206893 C should have a mass between ~ 8–15 M_Jup. Finally, further spectroscopic or photometric observations at higher signal-to-noise and longer wavelengths are required to learn more about the composition and dust cloud properties of HD 206893 B.