Fresh view of the hot brown dwarf HD 984 B through high-resolution spectroscopy

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
e-mail: jcostes01@qub.ac.uk
² Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
⁴ Department of Physics, University of Rome Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
⁵ INAF Osservatorio Astornomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, USA
⁸ Division of Geological & Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Department of Physics & Astronomy, 430 Portola Plaza, University of California, Los Angeles, CA 90095, USA
¹⁰ W. M. Keck Observatory, 65-1120 Mamalahoa Hwy, Kamuela, HI, USA
¹¹ Department of Astronomy & Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹² Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093, USA
¹³ Department of Astronomy, The Ohio State University, 100 W 18th Ave, Columbus, OH 43210, USA

Received: 24 October 2023
Accepted: 9 April 2024

Abstract

Context. High-resolution spectroscopy has the potential to drive a better understanding of the atmospheric composition, physics, and dynamics of young exoplanets and brown dwarfs, bringing clear insights into the formation channel of individual objects.

Aims. Using the Keck Planet Imager and Characterizer (KPIC; R « 35 000), we aim to characterize a young brown dwarf HD 984 B. By measuring its C/O and ¹²CO/¹³CO ratios, we expect to gain new knowledge about its origin by confirming the difference in the formation pathways between brown dwarfs and super-Jupiters.

Methods. We analysed the KPIC high-resolution spectrum (2.29–2.49 μm) of HD 984 B using an atmospheric retrieval framework based on nested sampling and petitRADTRANS, using both clear and cloudy models.

Results. Using our best-fit model, we find C/O = 0.50 ± 0.01 (0.01 is the statistical error) for HD 984 B which agrees with that of its host star within 1σ (0.40 ± 0.20). We also retrieve an isotopolog ¹²CO/¹³CO ratio of 98_-25⁺²⁰ in its atmosphere, which is similar to that of the Sun. In addition, HD 984 B has a substellar metallicity with [Fe/H] =-0.62_-0.02^+0.02. Finally, we find that most of the retrieved parameters are independent of our choice of retrieval model.

Conclusions. From our measured C/O and ¹²CO/¹³CO, the favored formation mechanism of HD 984 B seems to be via gravitational collapse or disk instability and not core accretion, which is a favored formation mechanism for giant exoplanets with m < 13 M_Jup and semimajor axis between 10 and 100 au. However, with only a few brown dwarfs with a measured ¹²CO/¹³CO ratio, similar analyses using high-resolution spectroscopy will become essential in order to determine planet formation processes more precisely.