The HUSTLE Program: The UV to Near-IR Transmission Spectrum of the Hot Jupiter KELT-7b

TL;DR

This study presents the UV to near-IR transmission spectrum of exoplanet KELT-7b, revealing disequilibrium chemistry and stellar surface inhomogeneities, demonstrating the importance of UV-optical data in atmospheric characterization.

Contribution

First to combine UV to near-IR spectra of KELT-7b, revealing disequilibrium chemistry and stellar heterogeneity, highlighting the value of UV observations for exoplanet atmosphere analysis.

Findings

High H- abundance indicating disequilibrium chemistry

Detection of asymmetric water feature in the spectrum

Evidence for bright stellar surface inhomogeneities

Abstract

The ultraviolet and optical wavelength ranges have proven to be a key addition to infrared observations of exoplanet atmospheres, as they offer unique insights into the properties of clouds and hazes and are sensitive to signatures of disequilibrium chemistry. Here we present the 0.2-0.8 $\mu$m transmission spectrum of the Teq = 2000 K Jupiter KELT-7b, acquired with HST WFC3/UVIS G280 as part of the HUSTLE Treasury program. We combined this new spectrum with the previously published HST WFC3/IR G141 (1.1-1.7 $\mu$m) spectrum and Spitzer photometric points at 3.6$\mu$m and 4.5$\mu$m, to reveal a generally featureless transmission spectrum between 0.2 and 1.7 $\mu$m, with a slight downward slope towards bluer wavelengths, and a asymmetric water feature in the 1.1-1.7 $\mu$m band. Retrieval models conclude that the 0.2 - 1.7$\mu$m spectrum is primarily explained by a high H- abundance ($\sim 10^{-5}$), significantly above the equilibrium chemistry prediction ($\sim 10^{-12}$), suggesting disequilibrium in KELT-7b's upper atmosphere. Our retrievals also suggest the presence of bright inhomogeneities in the stellar surface, and tentative evidence of CO2 at the Spitzer wavelengths. We demonstrate that with the UV-optical coverage provided by WFC3 UVIS/G280, we are able to confirm the presence and constrain the abundance of H-, and obtain evidence for bright stellar inhomogeneities that would have been overlooked using infrared data alone. Observations redward of 1$\mu$m with JWST should be able to further constrain the abundance of H-, as well as confirm the presence of CO2 inferred by the two Spitzer datapoints.