Orbit and atmosphere of HIP 99770 b through the eyes of VLTI/GRAVITY

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
² LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Department of Physics & Astronomy, Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Division of Space Research & Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
⁸ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁹ Fakultät für Physik, Universität Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany Universidade de Lisboa - Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
¹⁰ CENTRA - Centro de Astrofísica e Gravitaçâo, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
¹¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹² Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
¹³ Max Planck Institute for extraterrestrial Physics, Giessenbach-straße 1, 85748 Garching, Germany
¹⁴ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
¹⁵ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
¹⁶ STAR Institute, Université de Liège, Allée du Six Août 19c, 4000 Liège, Belgium
¹⁷ Department of Astrophysical & Planetary Sciences, JILA, Duane Physics Bldg., 2000 Colorado Ave, University of Colorado, Boulder, CO 80309, USA
¹⁸ Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
¹⁹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
²⁰ Universidade do Porto, Faculdade de Engenharia, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
²¹ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
²² Astrophysics Group, Department of Physics & Astronomy, University of Exeter, Stocker Road, Exeter EX4 4QL, UK
²³ Departments of Physics and Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
²⁴ European Southern Observatory, Casilla 19001, Santiago 19, Chile
²⁵ Advanced Concepts Team, European Space Agency, TEC-SF, ESTEC, Keplerlaan 1, NL-2201, AZ Noordwijk, The Netherlands
²⁶ University of Exeter, Physics Building, Stocker Road, Exeter EX4 4QL, UK
²⁷ French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and U. de Chile, Casilla 36-D, Santiago, Chile
²⁸ Center for Space and Habitability, University of Bern, Gesellschaftsstr. 6, 3012 Bern, Switzerland
²⁹ Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan
³⁰ European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
³¹ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
³² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
³³ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
³⁴ Excellence Cluster ORIGINS, Boltzmannstraße 2, 85748 Garching bei München, Germany

^★ Corresponding author.

Received: 26 March 2025
Accepted: 19 June 2025

Abstract

Context. Inferring the likely formation channel of giant exoplanets and brown dwarf companions from orbital and atmospheric observables remains a formidable challenge. Further and more precise directly measured dynamical masses of these companions are required to inform and gauge formation, evolutionary, and atmospheric models. We present an updated study of the recently discovered companion to HIP 99770 based on observations conducted with the near-infrared interferometer VLTI/GRAVITY.

Aims. Through renewed orbital and spectral analyses based on the GRAVITY data, we characterise HIP 99770 b to better constrain its orbit, dynamical mass, and atmospheric properties, as well as to shed light on its likely formation channel.

Methods. Upon inclusion of the new high-precision astrometry epoch, we ran an orbit fit to further constrain the dynamical mass of the companion and the orbit solution. We also analysed the GRAVITY K-band spectrum, placing it into context with literature data, and extracting magnitude, age, spectral type, bulk properties and atmospheric characteristics of HIP 99770 b.

Results. We detected the companion at a radial separation of 417 mas from its host. The new orbit fit yields a dynamical mass of 17₋₅⁺⁶ M_Jup and an eccentricity of 0.31_−0.12^+0.06. We also find that additional relative astrometry epochs in the future will not enable further constraints on the dynamical mass due to the dominating relative uncertainty on the Hipparcos-Gaia proper motion anomaly that is used in the orbit-fitting routine. The publication of Gaia DR4 will likely ease this predicament. Based on the spectral analysis, we find that the companion is consistent with spectral type L8 and exhibits a potential metal enrichment in its atmosphere. Adopting the AMES-DUSTY model to infer its age, within its dynamical mass constraint the companion conceivably corresponds to either a younger (28₋₁₄⁺¹⁵ Myr) object with a mass just below the deuterium-burning limit or an older (119₋₁₀⁺³⁷ Myr) body with a mass just above the deuterium-burning limit.

Conclusions. These results do not yet allow for a definite inference of the companion’s formation channel. Nevertheless, the new constraints on its bulk properties and the additional GRAVITY spectrum presented here will aid future efforts to determine the formation history of HIP 99770 b.