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

Abstract: 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 $v\sin i = 85.9\pm0.5$ km s$<sup>{-1}$. H$_2$O is strongly detected in all six targets and $<sup>{12}$CO in five, with a marginal $<sup>{12}$CO detection in the ultra-fast L0 rotator consistent with severe rotational broadening. $<sup>{13}$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 $<sup>{12}$C/$<sup>{13}$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 $<sup>{13}$CO cross-correlation peak. The M7 dwarf 2MASS J04341527+2250309 shows discrepant gravity and metallicity values between chemistry frameworks. Apparent H$_2<sup>{18}$O constraints for two targets are found to be spurious, and their H$_2<sup>{(16)}$O/H$_2<sup>{18}$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 $<sup>{12}$C/$<sup>{13}$C ratios at or above the ISM value, support a molecular cloud fragmentation origin for the sample. The agreement of $<sup>{12}$C/$<sup>{13}$C between chemistry frameworks supports the robustness of these ratios.