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Measurements of the C/O ratio in brown dwarfs are lacking, in part due topast models adopting solar C/O only. We have expanded the ATMO 2020 atmospheremodel grid to include non-solar metallicities and C/O ratios in the T dwarfregime. We change the C/O ratio by altering either the carbon or oxygenelemental abundances, and we find that non-solar abundances of these elementscan be distinguished based on the shapes of the $H$- and $K$- bands. We comparethese new models with medium-resolution ($R\approx1700$), near-infrared($0.8-2.4\,\mu$m) Gemini/GNIRS spectra of three benchmark late-T dwarfs, GJ570D, HD 3651B, and Ross 458C. We find solar C/O ratios and best-fittingparameters ($T_\mathrm{eff}$, $\log(g)$, $Z$) broadly consistent with otheranalyses in the literature based on low-resolution ($R\sim100$) data. Themodel-data discrepancies in the near-infrared spectra are consistent across allthree objects. These discrepancies are alleviated when fitting the Y, J, H andK bands individually, but the resulting best-fit parameters are inconsistentand disagree with the results from the full-spectrum. By examining the modelatmosphere properties we find this is due to the interplay of gravity andmetallicity on $\mathrm{H_2-H_2}$ collisionally induced absorption. Wetherefore conclude that there are no significant issues with the molecularopacity tables used in the models at this spectral resolution. Instead,deficiencies are more likely to lie in the model assumptions regarding thethermal structures. Finally, we find a discrepancy between the GNIRS, SpeX, andother near-infrared spectra in the literature of Ross 458C, indicatingpotential spectroscopic variability.

The Carbon-to-Oxygen Ratio in Cool Brown Dwarfs and Giant Exoplanets. I. The Benchmark Late-T dwarfs GJ 570D, HD 3651B and Ross 458C