Dayside Clouds and an Elevated C/O Ratio in the Atmosphere of the Ultrahot Jupiter WASP-19b

Abstract Ultrahot Jupiters (UHJs) offer exceptional opportunities for detailed atmospheric characterization via emission spectroscopy. We present a comprehensive analysis of the dayside atmosphere of WASP-19b—one of the shortest-period UHJs—using archival JWST NIRSpec/PRISM observations, leveraging its broad panchromatic wavelength coverage (0.6–5.2 μ m). Using atmospheric retrievals, we report robust detections of H 2 O (20.41 σ ) and CO (4.79 σ ), along with marginal evidence for CO 2 (2.11 σ ) and VO (2.81 σ ). Our retrievals also place strong constraints on the abundances of SiO and TiO, although their presence is not statistically confirmed. Furthermore, our analysis reveals strong evidence for clouds in this ultrahot dayside atmosphere (4.3 σ ), with indications of silicon dioxide (silica/quartz; SiO 2 (s)) cloud formation—making WASP-19b the first UHJ with a statistically significant cloud detection. Leveraging the well-constrained molecular abundances, we infer a dayside C/O ratio of 0.77 ± 0.16, a potentially supersolar value consistent with emerging trends among UHJs and suggestive of possible oxygen sequestration through cloud condensation. Our findings place WASP-19b as a key benchmark for modeling dayside atmospheric processes and evolutionary dynamics in extremely irradiated exoplanets.