Diagnosing limb asymmetries in hot and ultra-hot Jupiters with high-resolution transmission spectroscopy

TL;DR

This paper investigates how high-resolution transmission spectroscopy can diagnose atmospheric asymmetries in hot and ultra-hot Jupiters, focusing on Doppler shifts and chemical gradients to understand underlying atmospheric processes.

Contribution

It introduces diagnostic methods using models to identify causes of limb asymmetries, emphasizing the role of stable molecules like CO and the impact of clouds on observed spectra.

Findings

CO is an ideal baseline molecule due to its chemical stability.

Blueshifts decrease during transit if asymmetries are caused by morning clouds.

Binning spectra into two phase bins balances signal quality and asymmetry resolution.

Abstract

Due to their likely tidally synchronized nature, (ultra)hot Jupiter atmospheres should experience strongly spatially heterogeneous instellation. The large irradiation contrast and resulting atmospheric circulation induce temperature and chemical gradients that can produce asymmetries across the eastern and western limbs of these atmospheres during transit. By observing an (ultra)hot Jupiter's transmission spectrum at high spectral resolution, these asymmetries can be recovered -- namely through net Doppler shifts originating from the exoplanet's atmosphere yielded by cross-correlation analysis. Given the range of mechanisms at play, identifying the underlying cause of observed asymmetry is nontrivial. In this work, we explore sources and diagnostics of asymmetries in high-resolution cross-correlation spectroscopy of hot and ultra-hot Jupiters using both parameterized and self-consistent atmospheric models. If an asymmetry is observed, we find that it can be difficult to attribute it to equilibrium chemistry gradients because many other processes can produce asymmetries. Identifying a molecule that is chemically stable over the temperature range of a planetary atmosphere can help establish a ``baseline'' to disentangle the various potential causes of limb asymmetries observed in other species. We identify CO as an ideal molecule, given its stability over nearly the entirety of the ultra-hot Jupiter temperature range. Furthermore, we find that if limb asymmetry is due to morning terminator clouds, blueshifts for a number of species should decrease during transit. Finally, by comparing our forward models to Kesseli et al. (2022), we demonstrate that binning high-resolution spectra into two phase bins provides a desirable trade-off between maintaining signal to noise and resolving asymmetries.