Searching for the origin of the Ehrenreich effect in ultra-hot Jupiters: Evidence for strong C/O gradients in the atmosphere of WASP-76b?

Abstract

Extreme temperature contrasts between the day and nightside of ultra-hot Jupiters result in significantly asymmetric atmospheres, with a large expansion occurring over a small range of longitude around the terminator. Over the course of a transit, WASP-76 b rotates by about 30°, changing the observable part of the atmosphere and invoking variations in the appearance of its constituents. Specifically, during the latter part of the transit, the planet's trailing limb probes an increasing portion of its inflated dayside, which has a higher atmospheric detectability in transmission. As recently reported, this results in time-variable effects in the neutral iron signal, which are amplified by its possible condensation on the nightside. Here, we study the presence of molecular signals during a transit of WASP-76 b observed with the CARMENES spectrograph and compare the contributions from this planet's morning and evening terminators. The results are somewhat puzzling, with formal detections of water vapor (5.5σ) and hydrogen cyanide (5.2σ) but at significantly different positions in the K Psys diagram, with a blueshift of -14.3 ± 2.6 km s −1 and a redshift of +20.8 +7.8 −3.9 km s −1 respectively, and a higher K P than expected. The H 2 O signal also appears stronger later on in the transit, in contrast to that of HCN, which seems stronger early on. We tentatively explain this by silicate clouds forming and raining out on the nightside of the planet, partially removing oxygen from the upper atmosphere. For atmospheric C/O values between 0.7 and 1, this leads to the formation of HCN at the planet's morning limb. At the evening terminator, with the sequestered oxygen being returned to the gas phase due to evaporation, these C/O values lead to formation of H 2 O instead of HCN. Overall, if confirmed, these observations indicate that individual molecules trace different parts of the planet atmosphere, as well as nightside condensation, allowing spatial characterization. As these results are based on a single transit observation, we advocate that more data are needed to confirm these results and further explore these scenarios.