Uncovering 3D variations in the temperature, chemistry and winds of ultra-hot Jupiters: targeting multiple species with high-resolution spectroscopy

Ultra-hot Jupiters are tidally locked gas giants with dayside temperatures high enough to dissociate hydrogen and other molecules. Featuring sharp chemical gradients and large temperature contrasts, their atmospheres are vastly non-uniform. In recent years, the wealth of data from high-resolution spectrographs such as HARPS-N, CRIRES, ESPRESSO and IGRINS has yielded spectacular insights into the chemical inventory, wind profiles and temperature structures of ultra-hot Jupiters. In particular, high-precision observations with ESPRESSO were able to sample the varying Doppler shift of the absorption lines in the spectrum of a transiting exoplanet.

In this talk, I will present a new framework for computing time-dependent, 3D transit spectra of exoplanets at high spectral resolution. We post-process the output of a state-of-the-art global circulation model (the SPART/MITgcm) through a GPU-optimised Monte-Carlo radiative transfer code, called gCMCRT. This allows us to correctly model the millions of spectral lines that can be observed with ground-based instruments, while accounting for the complex effects of thermal and chemical inhomogeneities, wind gradients and planet rotation in a unified framework.

I will demonstrate that different chemical species, such as water, CO, and iron, are distributed differently throughout the atmosphere of an ultra-hot Jupiter, resulting in unique transit signals for certain atoms and molecules. This is because thermal dissociation and condensation impact the chemical composition of the dayside and the nightside of the planet. I will show that our framework can qualitatively reproduce the time-dependent iron signal observed by Ehrenreich+ (2020) for WASP-76b. To this end, we either assume that iron condenses into clouds on the nightside, or that the morning limb is substantially cooler than expected.

In addition, I will illustrate how signals from other species, such as water (which is dissociated on the dayside) and CO (which is unaffected by temperature) can be used to bypass the inherent degeneracies associated with the observation of a single species. Finally, I will cover the results of our recent transit observations of WASP-121b with Gemini-S/IGRINS. These observations targeted water and CO absorption in the infrared, with the objective to obtain a similar signal-to-noise ratio as the WASP-76b iron signal observed with ESPRESSO. I will discuss how interpreting the absorption signals of these three chemical species (water, CO, iron) within the same framework allows us to place important constraints on the 3D structure of the limb of an ultra-hot Jupiter.