Uniform Forward-modeling Analysis of Ultracool Dwarfs. I. Methodology and Benchmarking

We present a forward-modeling framework using the Bayesian inference tool Starfish and cloudless Sonora-Bobcat model atmospheres to analyze low-resolution ($R\approx80-250$) near-infrared ($1.0-2.5$ $\mu$m) spectra of T dwarfs. Our approach infers effective temperatures, surface gravities, metallicities, radii, masses, by accounting for uncertainties from interpolation correlated residuals due instrumental effects modeling systematics, produces more realistic parameter posteriors than traditional ($\chi^2$-based) spectral-fitting analyses. validate our fitting themselves finding negligible offsets between derived input parameters. apply methodology three well-known benchmark late-T dwarfs, HD 3651B, GJ 570D, Ross 458C, both solar non-solar metallicity atmospheric models. also derive these benchmarks' physical properties their bolometric luminosities, primary stars' ages evolutionary Assuming evolutionary-based parameters are robust, we find atmospheric-based, analysis two outcomes. For 3615B spectral fits provide accurate $T_{\rm eff}$ $R$ but underestimated $\log{g}$ (by $\approx1.2$ dex) $Z$ $\approx0.35$ dex), likely systematics potassium line profiles. overestimated $\approx120$ K) $\approx1.6\times$), because lack clouds, reduced vertical temperature gradients, or disequilibrium processes. Finally, spectroscopically inferred masses benchmarks all considerably underestimated.