Atmospheric Retrievals of Brown Dwarf Binary Luhman 16AB: C/O Ratios and Systematic Uncertainties

  Brown dwarfs, which share atmospheric properties with gas giant exoplanets and form via star-like processes, serve as important benchmarks for understanding the relation between atmospheric composition and the formation processes of substellar objects. While the carbon-to-oxygen (C/O) ratio in their atmospheres is expected to reflect the formation history, measurements of their C/O ratios remain limited, and systematic uncertainties in retrieval analyses, such as molecular line lists and temperature-pressure (T-P) profile parameterizations, are not yet fully understood.
  Here we present C/O measurements of the benchmark brown dwarf binary Luhman 16AB and quantify systematic uncertainties due to line lists and T-P profile parameterizations using high-resolution atmospheric retrievals of VLT/CRIRES spectra with the ExoJAX framework. We find C/O ratios of ~0.67 for both components, slightly above solar, with line list systematics at the ~7% level dominating the uncertainty budget. We also explore Gaussian process-based T–P profiles and demonstrate their utility for flexible thermal structure modeling. These results establish Luhman 16AB as a key anchor for substellar C/O measurements and highlight the importance of systematic effects in comparative studies of substellar objects.