Benchmark Brown Dwarfs as Chemical Laboratories: Linking System Bulk Properties to Atmospheric Retrievals

We present the first atmospheric retrieval analysis of two compositional benchmark mid-L dwarfs – SDSSJ141659.78+500626.4 and GJ 499 C – using the Brewster retrieval framework, where the wide benchmark nature of these systems provides independent constraints on age and bulk composition from their stellar primaries. These targets were observed with JWST using NIRSpec Prism and MIRI LRS, providing low-resolution (R ~ 100) spectra between 0.6–14.0 μm with high signal-to-noise ratios (SNR ~100 – 600). For SDSSJ141659.78+500626.4, the retrieved cloud combination of a high-altitude enstatite slab and low-altitude iron deck clouds matches phase-equilibrium predictions based on the primary star’s Mg/Si ratio. We retrieve a super-solar C/O = 0.71+0.01-0.01 and slightly metal-rich [M/H] = 0.22+0.03-0.03. This C/O ratio can only be reconciled with the value inferred for the primary if additional oxygen sequestration beyond the retrieved cloud mass is present, or if there are uncertainties in the adopted opacities or other model deficiencies. The inferred [C/H] and [O/H] are 0.31±0.03 and 0.19±0.03, respectively, which are consistent within the relatively large uncertainties of the host star abundances. For GJ 499 C, the retrieved silicon-monoxide and forsterite slab clouds are difficult to explain with simple phase-equilibrium assumptions, yielding Mg/Si ~ 1.9. We estimate C/O = 0.69+0.02-0.02 and [M/H] = 0.13+0.04-0.06. For both objects, the inferred radii of 0.85+0.01-0.01 RJup and 1.00+0.02-0.02 RJup and masses of 73.7+4.9-9.2 MJup and 68.0+8.5-12.7 MJup are consistent with evolutionary models and system ages, highlighting the plausibility of our results.