Chemistry and Isotope Ratios of Substellar Atmospheres in the β Pictoris Young Moving Group and Vicinity

Abstract Measuring the chemical and isotopic compositions of gas giants and brown dwarfs provides insights into their formation pathways and birth environments. 2MASS J0249-0557c is an L2-type planetary mass companion (∼12 M Jup ) orbiting a pair of brown dwarfs in the β Pictoris young moving group and vicinity. 7 7 While E. L. Shkolnik et al. (2017) and T. J. Dupuy et al. (2018) have attributed 2MASS J0249-0557AB to the β Pic young moving group, work by K. L. Luhman (2024) has attributed 2MASS J0249-0557AB to the 32 Ori young moving group, which spatially overlaps with the β Pic young moving group. The two groups have been proposed to be physically connected (J. Gagné et al. 2021). Its mass places it at the intersection of planets and brown dwarfs, making it an interesting target for constraining formation pathways at the planet–brown dwarf boundary. Using high-resolution spectroscopic data of the planet acquired with CRIRES+ mounted on the Very Large Telescope, we conduct atmospheric retrieval with the radiative transfer code petitRADTRANS and the nested sampling tool PyMultiNest. We retrieve a C/O ratio of 0.57 ± 0.01, a metallicity of [M/H] = 0.18 ± 0.05, and a 12 CO/ 13 CO ratio of 9 5 − 17 + 23 . We also retrieve atmospheric compositions for two benchmark brown dwarfs in the β Pic YMG, 2MASSI J0443+0002 and SIPS J2000-7523, using CRIRES+ data and find consistent compositions. Together with 2MASS J0249-0557 c’s wide separation from its host, its compositional consistency with benchmark brown dwarfs supports gravitational collapse in a star-like manner as its most likely formation mechanism. These results deliver a homogeneous comparison of three substellar members in the β Pic YMG and vicinity. Their solar-like abundances provide a baseline for exoplanet members in the same moving group, such as β Pic b, 51 Eri b, and AF Lep b, whose host stellar compositions are difficult to measure. Future comparisons of atmospheric compositions among this moving group offer the potential to distinguish between formation mechanisms for its planetary members.