The ESO SupJup Survey XI. Atmospheric properties of six isolated M- and L-type dwarfs with CRIRES+

The distinct formation pathways of brown dwarfs and giant exoplanets may be encoded in their atmospheric composition. We present atmospheric retrievals for six isolated brown dwarfs of spectral types M7-L2.5 from the ESO SupJup Survey, aiming to constrain their thermal structures, chemical compositions, and isotope ratios. We analyse CRIRES+ K-band spectra, coupling the radiative transfer code petitRADTRANS with the nested sampling algorithm PyMultiNest under both free and equilibrium chemistry frameworks. The L0 dwarf 2MASS J09532126-1014205 emerges as one of the fastest-rotating ultracool dwarfs known, with v i = 85.9 plus or minus0.5 km s-1. H 2 O is strongly detected in all six targets and 12 CO in five, with a marginal 12 CO detection in the ultra-fast L0 rotator consistent with severe rotational broadening. 13 CO is significantly detected in DENIS J060852.8-275358 and tentatively in three further targets. Retrieved compositions are consistent with isolated brown dwarfs: near-solar C/O ratios (0.51 - 0.63), predominantly near-solar metallicities, and 12 C/ 13 C ratios of 91-155, at or above the local ISM value, with constraints for the two fastest rotators resting on the spectral fit but not corroborated by a 13 CO cross-correlation peak. The M7 dwarf 2MASS J04341527+2250309 shows discrepant gravity and metallicity values between chemistry frameworks. Apparent H 2 18 O constraints for two targets are found to be spurious, and their H 2(16) O/H 2 18 O ratios are presented as lower limits, demonstrating the importance of cross-correlation validation for minor species detections. The near-solar C/O ratios and metallicities, with 12 C/ 13 C ratios at or above the ISM value, support a molecular cloud fragmentation origin for the sample. The agreement of 12 C/ 13 C between chemistry frameworks supports the robustness of these ratios.