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

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.