Uniform Forward-Modeling Analysis of Ultracool Dwarfs. IV. Benchmarking the Sonora Diamondback and Saumon & Marley (2008) Atmospheric Models Across Late-M, L, and T types with Low-Resolution 0.8-2.5 $μ$m Spectroscopy

TL;DR

This paper evaluates the performance of two major cloudy atmospheric model grids, SM08 and Sonora Diamondback, by forward-modeling a uniform, large sample of 142 age-benchmark ultracool dwarfs with low-resolution SpeX 0.8–2.5 μm spectra. By anchoring atmospheric inferences to Sonora Diamondback evolution models and comparing to evolution-based properties, the authors quantify systematic discrepancies in $T_{ m eff}$, $R$, and $\log g$, and reveal a population-level age dependence of the cloud sedimentation parameter $f_{ m sed}$ for L4–L9 dwarfs. They provide empirical calibrations linking atmospheric-fit parameters to evolution-based quantities, identify persistent wavelength-dependent residuals tied to key opacities, and show that including an ISM-like extinction term improves fits—highlighting missing opacity sources and cloud physics in current models. The work underscores ongoing challenges in modeling cloudy ultracool atmospheres and outlines concrete paths (opacities, rainout chemistry, extinction treatments) to improve atmospheric-model fidelity for brown dwarfs and directly imaged exoplanets. These results offer essential context for interpreting atmospheric parameters in future large surveys and benchmark-based studies.

Abstract

(Abridged) We present a systematic assessment of two major cloudy atmospheric model grids -- SM08 (Saumon & Marley 2008) and Sonora Diamondback -- when applied to low-resolution near-infrared (0.8-2.5 $μ$m) spectroscopy. Our analysis focuses on a uniform sample of 142 age-benchmark brown dwarfs and planetary-mass objects spanning late-M, L, and T spectral types, with independently determined ages from 10 Myr to 10 Gyr. We perform forward-model spectral fitting for all benchmarks' IRTF/SpeX spectra ($R\sim$80-250) using both SM08 and Sonora Diamondback atmospheric models to infer effective temperatures, surface gravities, metallicities, radii, and cloud sedimentation efficiencies. The two model grids yield broadly consistent results. Among L4-L9 dwarfs, we identify a statistically significant, population-level age dependence of the cloud parameter $f_{\rm sed}$, with young benchmarks ($<300$ Myr) exhibiting systematically lower $f_{\rm sed}$ values than older counterparts. This trend is absent across L0-T5 and T0-T5, demonstrating that cloud properties vary with age and surface gravity and offering explanations for the observed gravity-dependent photometric properties at the late-L end of the L/T transition. By comparing spectroscopically inferred parameters with predictions from evolution models, we quantify systematic errors in the fitted atmospheric parameters and establish empirical calibrations to anchor future studies using these atmospheric models. Stacked residuals of the sample reveal wavelength-dependent data-model mismatches associated with key atomic and molecular absorption bands, highlighting the need for improved opacities and rainout chemistry. Finally, we show that including an interstellar-medium-like extinction term significantly improves the spectral fits, confirming and broadening previous findings and suggesting missing opacity sources in current cloudy models.

Figures (20)

• Figure 1: Spectral type distribution of our sample of benchmark brown dwarfs and planetary-mass objects, with ages younger (blue) or older (orange) than 300 Myr.
• Figure 2: $J$-band absolute magnitudes and $J-K$ colors of our sample. Young benchmarks with ages below 300 Myr are shown as blue circles, while those older than 300 Myr are shown in orange. We overlay previously known late-M, L, T, and Y dwarfs from the UltracoolSheet ultracoolsheet; these objects have precise photometry (S/N$>5$) and are neither candidate nor resolved binaries.
• Figure 3: Ages and bolometric luminosities of our benchmark targets (Table \ref{['tab:evo']}), color-coded by spectral types. We use open circles to mark the benchmarks excluded from the evolution model analysis, including binaries and several objects whose ages and/or $L_{\rm bol}$ fall outside the convex hull of the evolution models (see Section \ref{['sec:evo']}). The Sonora Diamondback hybrid-grav evolution models (black lines) with [M/H]$=0$ dex are overlaid.
• Figure 4: $^{2}$-based spectral fitting results of three representative targets based on the Sonora Diamondback atmospheric models. In the left panels, the observed IRTF/SpeX spectra (black) are compared with the best-fit model spectra (red), and the corresponding best-fit parameters are labeled. Fluxes within the broad water bands (shaded in gray) are masked in spectral fits. The right panels present the $^{2}$ values as a function of effective temperatures, with variations in symbol, color, and transparency indicating different values of $\log{(g)}$, $f_{\rm sed}$, and [M/H], respectively.
• Figure 5: Best-fit synthetic spectra for all targets using the SM08 models (blue) and Sonora Diamondback (red) models, overlaid with the observed IRTF/SpeX spectra (black). Fluxes within the broad water bands (shaded in gray) are masked in spectral fits. Targets are ordered by spectral types, spanning M7 to L3.