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

Abstract

Ultra-hot Jupiters (UHJs) offer exceptional opportunities for detailed atmospheric characterization via emission spectroscopy. 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, 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 ultra-hot dayside atmosphere (4.3 $σ$), with indications of silicon dioxide (silica/quartz; SiO$_2$(s)) cloud formation$\unicode{x2014}$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 $\pm$ 0.16, a potentially super-solar 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.

Figures (12)

• Figure 1: Top: Spectroscopic planet-to-star flux ratios with 1$\sigma$ uncertainties, derived from two independent reductions of the NIRSpec/PRISM data using Eureka! and exoTEDRF, followed by the modeling of the spectroscopic secondary eclipse lightcurves. The white lightcurve and the 196th spectroscopic lightcurve (Eureka!, both binned to 2-minute cadence for clarity) are shown along with their respective best-fit secondary-eclipse models. Bottom: Planetary emission spectra, normalized to the observer's location, derived from the planet-to-star flux ratios above and modeled PHEONIX stellar spectrum. The mean absolute difference between the two independently reduced spectra is $\sim$1$\sigma$, indicating excellent statistical consistency. For reference, blackbody curves corresponding to 2600K, 2400K, 2200K, and 2000K are also shown. The significant deviation of the planet's spectra from blackbody expectations at shorter wavelengths is due to the dominating contribution of the reflected star-light by the planet.
• Figure 2: The observed emission spectrum of WASP-19b in terms of brightness temperatures (orange, Eureka!) is shown, along with the best-fit free-chemistry retrieved model and the residuals. Retrieved models excluding individual species are also shown, which were used to assess the statistical significance of their detection. Key spectral features from H$_2$O, CO, and CO$_2$, each detected with strong statistical significance, are also highlighted for reference.
• Figure 3: The retrieved posterior P–T profiles from the free (left) and equilibrium chemistry (right) retrievals of the Eureka! spectrum are shown, along with the corresponding SiO$_2$(s) condensation curves and their abundance profiles.
• Figure 4: The retrieved median abundances of H$_2$O, CO, CO$_2$, SiO, TiO, and VO are shown from both free (solid) and equilibrium (dashed-dotted) retrievals for comparison (Eureka!), along with the median contribution function from the free chemistry retrievals. The strong contribution from SiO$_2$(s) cloud is apparent.
• Figure 5: Several reported C/O ratios for a wide range of exoplanets based on JWST observations are shown, plotted against their equilibrium temperatures, with the estimated C/O ratio for WASP-19b from this work highlighted as the magenta bowtie. A step-function model illustrating the potential onset of super-solar C/O ratios in UHJs is shown, along with a quadratic polynomial model representing a gradual increase of the C/O ratio with equilibrium temperature, both with their corresponding 1$\sigma$ uncertainties.