ExoJAX Retrievals of VLT/CRIRES Spectra of Luhman 16AB: C/O Ratios and Systematic Uncertainties

TL;DR

This study performs atmospheric retrievals on Luhman 16AB using high-resolution spectra, assessing systematic uncertainties in C/O ratios and temperature profiles, and emphasizes the importance of flexible modeling and systematic testing for brown dwarf characterization.

Contribution

It introduces a flexible Gaussian process-based temperature-pressure profile in high-resolution retrievals and systematically evaluates uncertainties, especially from molecular line lists.

Findings

C/O ratios around 0.67, slightly above solar

Line list uncertainties dominate systematic errors

Flexible T-P profiles yield consistent atmospheric properties

Abstract

We present atmospheric retrievals of the benchmark brown dwarf binary Luhman 16AB using high-resolution VLT/CRIRES spectra and the differentiable framework ExoJAX. We derive elemental abundances and temperature-pressure ($T$-$P$) profiles while explicitly testing the robustness of the results against major sources of systematic uncertainty. We first perform retrievals with a power-law $T$-$P$ profile and assess the sensitivity of inferred molecular abundances and C/O ratios to different CO line lists (ExoMol, HITEMP with air- and H2-broadening). We then introduce a flexible Gaussian process-based $T$-$P$ profile, allowing a non-parametric characterization of the thermal structure and a more conservative treatment of uncertainties. For both components, we infer C/O ratios of about 0.67, slightly above solar, with line list systematics at the 7 percent level emerging as the dominant source of uncertainty, whereas assumptions about $T$-$P$ parameterization or photometric variability play a lesser role. The retrieved $T$-$P$ profiles and molecular abundances are broadly consistent with atmospheric models and equilibrium chemistry. Our results establish Luhman 16AB as a key anchor for substellar C/O measurements, demonstrate the utility of flexible $T$-$P$ modeling in high-resolution retrievals, and highlight the importance of systematic tests -- particularly line list uncertainties -- for robust comparisons between brown dwarfs and giant exoplanets.