2024A&A...686A.294C

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.