The KELT-7b atmospheric thermal-inversion conundrum revisited with CHEOPS, TESS, and additional data

TL;DR

This study revisits the atmospheric properties of ultrahot Jupiter KELT-7b using CHEOPS, TESS, and literature data, revealing a non-inverted temperature-pressure profile with model-dependent interpretations and low albedo, highlighting the complexity of atmospheric characterization.

Contribution

The paper provides a comprehensive reanalysis of KELT-7b's atmosphere using multiple datasets and modeling approaches, emphasizing the impact of chemical assumptions on temperature profile inference.

Findings

Thermochemical equilibrium models suggest a non-inverted T-P profile.

Free-chemistry retrievals indicate an inverted profile with unphysical TiO/VO levels.

Low geometric albedo of approximately 0.05, implying inefficient heat redistribution.

Abstract

Ultrahot Jupiters are predicted to show inverted temperature-pressure (T-P) profiles in the presence of optical absorbers such as TiO and VO. An inverted T-P profile of KELT-7b was recently detected, in line with these predictions, but such diagnoses are known to be model-dependent. We used CHEOPS, TESS, and literature data to characterize the atmosphere of KELT-7b, reassess its T-P profile, measure its albedo, and search for distortions in its CHEOPS transit light curve due to stellar rotation. We jointly fitted CHEOPS and TESS data to measure the occultation depths and modeled CHEOPS transits including gravity darkening. Emission and transmission retrievals were performed, and the albedo was calculated in the CHEOPS and TESS passbands. Thermochemical-equilibrium retrievals yield a non-inverted T-P profile, while free-chemistry retrievals yield an inverted profile with likely unphysical TiO/VO abundances. A 3D GCM supports a TiO-driven inversion. We report a low geometric albedo of $A_\mathrm{g} = 0.05 \pm 0.06$, consistent with inefficient heat redistribution and supported by a GCM with magnetic drag. CHEOPS data provide no constraint on the sky-projected orbital obliquity. Retrieval results strongly depend on the chemical framework. Free-chemistry fits are better but risk unphysical solutions for ultrahot Jupiters. We applied a coherent stellar variability correction to CHEOPS and TESS data; future observations would benefit from similar treatment.