The effects of stellar gravity darkening on high-resolution transmission spectra

TL;DR

This paper investigates how stellar gravity darkening influences high-resolution transmission spectra of exoplanets, showing it can alter absorption line shapes and must be considered in future observations with large telescopes.

Contribution

It demonstrates the impact of stellar gravity darkening on transmission spectra and highlights its importance for accurate atmospheric characterization with upcoming large telescopes.

Findings

Gravity darkening affects the shape of absorption light curves.

The effect is comparable to noise levels in current 10-meter class observations.

Future 30-meter class telescopes must account for gravity darkening effects.

Abstract

High-resolution transmission spectroscopy is a powerful method for probing the extended atmospheres of short-period exoplanets. With the advancement of ultra-stable echelle spectrographs and the advent of 30-meter class telescopes on the horizon, even minor observational and physical effects will become important when modeling atmospheric absorption of atomic species. In this work we demonstrate how the non-uniform temperature across the surface of a fast rotating star, i.e., gravity darkening, can affect the observed transmission spectrum in a handful of atomic transitions commonly observed in short-period exoplanet atmospheres. We simulate transits of the ultra-hot Jupiters KELT-9 b and HAT-P-70 b but our results are applicable to all short-period gas giants transiting rapidly rotating stars. In general, we find that gravity darkening has a small effect on the average transmission spectrum but can change the shape of the absorption light curve, similar to the effect observed in broadband photometric transits. While the magnitude of gravity darkening effects are on the same order as the noise in transmission spectra observed with 10-meter class telescopes, future high-quality spectroscopic light curves for individual atomic absorption lines collected with 30-meter class telescopes will need to account for this effect.