A Highly Carbon-Rich Dayside and Disequilibrium Chemistry in the Ultra-Hot Jupiter WASP-19b

Abstract: Ultra-hot Jupiters (UHJs) offer exceptional opportunities for detailed atmospheric characterization via emission spectroscopy. Here we present a comprehensive analysis of the dayside atmosphere of WASP-19b$\unicode{x2014}$one of the shortest-period UHJs$\unicode{x2014}$using archival JWST NIRSpec/PRISM observations spanning 0.6-5.3 $\mu$m. We report robust detections of H$2$O (16.44 $\sigma$), CO (5.47 $\sigma$), and CO$_2$ (10.72 $\sigma$), along with marginal detections of CH$_4$ (3.76 $\sigma$) and C$_2$H$_2$ (2.45 $\sigma$). The retrieved composition reveals a highly carbon-rich atmosphere with a tightly constrained super-solar C/O ratio of 0.94$\pm$0.03. Elevated abundances of carbon-bearing species provide strong evidence (11.69 $\sigma$) for disequilibrium chemistry. We also detect condensate clouds, likely Al$_2$O$_3$(c), at high significance (17.28 $\sigma$), and constrain the atmospheric metallicity to 1.7${-0.7}^{+1.2}$ $\times$ solar. These results establish a precise benchmark for modeling dayside conditions in extreme irradiated atmospheres and demonstrate JWST's transformative capabilities for exoplanet science.