The Emission Spectrum of the Hot Jupiter WASP-79b from HST/WFC3

TL;DR

This study presents the thermal emission spectrum of the hot Jupiter WASP-79b from HST/WFC3 observations, compares it with atmospheric models, and discusses the implications for its atmospheric composition and properties.

Contribution

First measurement of WASP-79b's emission spectrum in the 1.1-1.7 μm range using HST/WFC3, combined with Spitzer data, and analysis of atmospheric models to interpret its atmospheric characteristics.

Findings

WASP-79b's brightness temperature is approximately 1900 K.

Multiple atmospheric models, including cloudy and disequilibrium chemistry, fit the data.

Best-fit models suggest solar metallicity and C/O ratio with specific recirculation factors.

Abstract

Here we present a thermal emission spectrum of WASP-79b, obtained via Hubble Space Telescope Wide Field Camera 3 G141 observations as part of the PanCET program. As we did not observe the ingress or egress of WASP-79b's secondary eclipse, we consider two scenarios: a fixed mid-eclipse time based on the expected occurrence time, and a mid-eclipse time as a free parameter. In both scenarios, we can measure thermal emission from WASP-79b from 1.1 to 1.7 $\mu$m at 2.4$\sigma$ confidence consistent with a 1900 K brightness temperature for the planet. We combine our observations with Spitzer dayside photometry (3.6 and 4.5 $\mu$m) and compare these observations to a grid of atmospheric forward models that span a range of metallicities, carbon-to-oxygen ratios, and recirculation factors. Given the strength of the planetary emission and the precision of our measurements, we found a wide range of forward models to be consistent with our data. The best-match equilibrium model suggests that WASP-79b's dayside has a solar metallicity and carbon-to-oxygen ratio, alongside a recirculation factor of 0.75. Models including significant H- opacity provide the best match to WASP-79b's emission spectrum near 1.58 $\mu$m. However, models featuring high-temperature cloud species-formed via vigorous vertical mixing and low sedimentation efficiencies-with little day-to-night energy transport also match WASP-79b's emission spectrum. Given the broad range of equilibrium chemistry, disequilibrium chemistry, and cloudy atmospheric models consistent with our observations of WASP-79b's dayside emission, further observations will be necessary to constrain WASP-79b's dayside atmospheric properties.