Phase-resolved asymmetries of (ultra)hot Jupiters in high-resolution transmission: drivers and diagnostics

Exoplanet atmospheres are inherently three-dimensional. While assuming otherwise is often justified by data quality, current observations are rapidly approaching — and in some cases have already reached — data quality that requires a fuller treatment. Notably, high-resolution spectroscopy has recently yielded a slew of high-precision datasets for (ultra)hot Jupiters. Many of these datasets indicate the presence of three-dimensional (3D) thermal, dynamical, and/or chemical asymmetries in these planets’ atmospheres. However, the relative dominance of the mechanisms invoked to explain these asymmetries in some cases remains unclear. This work investigates several key proposed drivers of asymmetry in (ultra)hot Jupiter atmospheres and determines their impact on high-resolution transmission spectra. We furthermore evaluate the effectiveness of diagnostics such as net Doppler shifts measured during transit and ingress/egress differences to probe the 3D nature of an exoplanet’s atmosphere, in addition to comparing the extent of an extended evening limb’s excess abundance with respect to the effect of limb-to-limb chemical equilibrium variations. Additionally, experiments post-processing a WASP-76b general circulation model imply that with current error budgets, combining Doppler shifts into two phase bins only reliably reveals east-west asymmetries when ingress and egress phases are included. We demonstrate, moreover, that the thermochemical stability of CO over a broad temperature/pressure regime makes this molecule an excellent standard for Doppler asymmetry studies, because it provides a baseline for the Doppler shifts obtained from a species that is uniformly distributed over the atmosphere. Finally, motivated by near-infrared observations of ultrahot Jupiters, we aim to determine what can be learned about atmospheric dynamics and carbon/oxygen partitioning by comparing the spectral and cross-correlation signatures of three key absorbers (H₂O, CO, and OH). With current best error bars, only the strongest effect can be robustly recovered: that H₂O forms at higher altitudes than CO, allowing H₂O to probe day-night flow and produce a stronger blueshift than CO.