From pre-transit to post-eclipse: investigating the impact of 3D temperature, chemistry, and dynamics on high-resolution emission spectra of the ultra-hot Jupiter WASP-76b

TL;DR

This study models phase-dependent emission spectra of ultra-hot Jupiter WASP-76b using 3D simulations to understand how atmospheric dynamics affect spectral line shifts and detectability of key molecules.

Contribution

It introduces a comprehensive 3D forward-modeling approach to interpret high-resolution spectra of ultra-hot Jupiters, revealing the impact of rotation and temperature structure on spectral features.

Findings

Negative $K_{p}$ offsets are driven by planet rotation.

Weak vertical temperature gradients mute certain molecular features.

Offsets in spectral lines are influenced by blended emission and absorption signals.

Abstract

High-resolution spectroscopy has provided a wealth of information about the climate and composition of ultra-hot Jupiters. However, the 3D structure of their atmospheres makes observations more challenging to interpret, necessitating 3D forward-modeling studies. In this work, we model phase-dependent thermal emission spectra of the archetype ultra-hot Jupiter WASP-76b to understand how the line strengths and Doppler shifts of Fe, CO, H$_2$O, and OH evolve throughout the orbit. We post-process outputs of the SPARC/MITgcm global circulation model with the 3D Monte-Carlo radiative transfer code gCMCRT to simulate emission spectra at 36 orbital phases. We then cross-correlate the spectra with different templates to obtain CCF and $K_{\text{p}}$$-$$V_{\text{sys}}$ maps. For each species, our models produce consistently negative $K_{\text{p}}$ offsets in pre- and post-eclipse, which are driven by planet rotation. The size of these offsets is similar to the equatorial rotation velocity of the planet. Furthermore, we demonstrate how the weak vertical temperature gradient on the nightside of ultra-hot Jupiters mutes the absorption features of CO and H$_2$O, which significantly hampers their detectability in pre- and post-transit. We also show that the $K_{\text{p}}$ and $V_{\text{sys}}$ offsets in pre- and post-transit are not always a measure for the line-of-sight velocities in the atmosphere. This is because the cross-correlation signal is a blend of dayside emission and nightside absorption features. Finally, we highlight that the observational uncertainty in the known orbital velocity of ultra-hot Jupiters can be multiple km/s, which makes it hard for certain targets to meaningfully report absolute $K_{\text{p}}$ offsets.