KELT-20b/MASCARA-2b CHEOPS light curve

We interpret the occultation measurements together with archival infrared observations to measure the planetary geometric albedo and dayside temperatures. We further use the host star's gravity-darkened nature to measure the system's obliquity. We present time-averaged precise occultation depth of 82+/-6ppm measured with seven CHEOPS visits and 131^+8^_-7_ ppm from the analysis of all available TESS photometry.

Using these measurements, we precisely constrain the geometric albedo of KELT-20 b to 0.26+/-0.04 and the brightness temperature of the dayside hemisphere to 2566^+77^_-80_K. Assuming Lambertian scattering law, we constrain the Bond albedo to 0.36^+0.04^_-0.05_ along with a minimal heat transfer to the nightside (<=0.14^+0.13^_-0.10_). Furthermore, using five transit observations we provide stricter constraints of 3.9+/-1.1-degrees on the sky-projected obliquity of the system.

Conclusions. The aligned orbit of KELT-20 b is in contrast to previous CHEOPS studies that have found strongly inclined orbits for planets orbiting other A-type stars. KELT-20 b's comparably high planetary geometric albedo corroborates a known trend of strongly irradiated planets being more reflective.

Finally, we tentatively detect signs of temporal variability in the occultation depths, which might indicate variable cloud cover advecting onto the planetary dayside.