Precise Determination of the Metallicity and C/O of WASP-39~b From a Single JWST Instrument Mode with Phase-Resolved Cross-Correlation Retrievals

Abstract

Measuring atmospheric metallicities and C/O ratios is a key goal of JWST exoplanet science, given their proposed link to planet formation. Achieving this goal has previously been shown to require broad wavelength coverage ($\sim$1--5 $\mu$m), typically demanding multiple instrument modes to complete the molecular inventory. Here, we show that the multi-instrument requirement can be circumvented using phase-resolved cross-correlation retrievals at native pixel resolution -- an approach more typically applied to ground-based high-resolution spectroscopy. By applying this novel analysis technique to an archival single-mode transit of the hot Jupiter WASP-39~b obtained with NIRSpec/G395H, we detect and obtain bounded abundances for all of its major carbon- and oxygen-bearing molecules: H${}_2$O ($\Delta \ln (Z)=67$), CO ($\Delta \ln (Z)=25$), CO${}_2$ ($\Delta \ln (Z)=475$), and SO${}_2$ ($\Delta \ln (Z)=10$). Notably, while standard retrieval methods fail to detect CO in these same data ($\Delta \ln (Z)=0.2$), our approach detects it decisively, confirming it as the dominant carbon carrier in WASP-39 b's atmosphere. From these abundances, we robustly derive a metallicity of $[(\mathrm{C}+\mathrm{O})/H]=1.2\pm 0.2$ and a C/O ratio of $0.68_{-0.14}^{+0.10}$, generally consistent with previous multi-instrument analyses. In comparison, traditional retrievals performed on the G395H data alone produce biased and inaccurate values of both parameters, driven primarily by the non-detection of CO as well as incomplete water band coverage. Our results establish phase-resolved cross-correlation retrievals as a powerful tool for extracting maximum atmospheric information from existing and future JWST data sets.

Submitted to AAS Journals. 16 pages, 7 figures, 1 table. Comments welcome!