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

Measuring the chemical and isotopic compositions of gas giants and brown dwarfs provides insights into their formation pathways and birth environments. 2MASS J0249-0557 c is an L2-type planetary mass companion (about 12 M Jup) orbiting a pair of brown dwarfs in the beta Pic young moving group and vicinity. 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 VLT, 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 plus or minus0.01, a metallicity of [M/H] = 0.18 plus or minus0.05, and a 12 CO/ 13 CO ratio of 95+23-17. We also retrieve atmospheric compositions for two benchmark brown dwarfs in the beta 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 beta Pic YMG and vicinity. Their solar-like abundances provide a baseline for exoplanet members in the same moving group, such as beta 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.