CHEOPS and TESS photometry of KELT-7

The CHEOPS transits were also fitted with a model including the gravity-darkening effect. Emission and absorption retrievals were performed to characterize the atmosphere of KELT-7b. The albedo of the planet was calculated in the CHEOPS and TESS passbands.

When adopting a thermochemical-equilibrium atmospheric composition, the emission retrievals return a non-inverted T-P profile, in contrast with previous results. When adopting a free-chemistry atmospheric parameterization, the emission retrievals return an inverted T-P profile with -- likely unphysically -- high concentrations of TiO and VO. The 3D general circulation model (GCM) supports a TiO-induced temperature inversion.

We report for KELT-7b a very low geometric albedo of A_g_=0.05+/-0.06, which is consistent with the heat distribution {epsilon} being close to zero and also consistent with a 3D GCM simulation, using magnetic drag ({tau}_drag_=10^4^s). Based on the CHEOPS photometry, we are unable to place any meaningful constraint on the sky-projected orbital obliquity. The choice of a free-chemistry approach or a thermochemical-equilibrium chemistry is the main factor determining the retrieval results.

Free-chemistry retrievals generally yield better fits; however, assuming free chemistry risks adopting unphysical scenarios for ultrahot Jupiters, such as KELT-7b. We applied a coherent stellar variability treatment on TESS and CHEOPS observations, commensurate with the known stellar activity of the host star. Other observations of KELT-7b would also benefit from a coherent stellar variability treatment.