Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS) - II. A broadened sodium feature on the ultra-hot giant WASP-76b

TL;DR

This study uses high-resolution transit spectroscopy to detect and analyze a broadened sodium absorption feature in the ultra-hot exoplanet WASP-76b's atmosphere, suggesting super-rotation effects.

Contribution

First detection of a broadened sodium line profile in WASP-76b's atmosphere, indicating atmospheric super-rotation in an ultra-hot gas giant.

Findings

Neutral sodium detected with high significance (10.75σ).

Sodium lines are significantly broader than the instrument profile.

Broadening suggests super-rotation in the planet's upper atmosphere.

Abstract

High-resolution optical spectroscopy is a powerful tool to characterise exoplanetary atmospheres from the ground. The sodium D lines, with their large cross sections, are especially suited to study the upper layers of atmospheres in this context. We report on the results from HEARTS, a spectroscopic survey of exoplanet atmospheres, performing a comparative study of hot gas giants to determine the effects of stellar irradiation. In this second installation of the series, we highlight the detection of neutral sodium on the ultra-hot giant WASP-76b. We observed three transits of the planet using the HARPS high-resolution spectrograph at the ESO 3.6m telescope and collected 175 spectra of WASP-76. We repeatedly detect the absorption signature of neutral sodium in the planet atmosphere ($0.371\pm0.034\%$; $10.75 \sigma$ in a $0.75$ \r{A} passband). The sodium lines have a Gaussian profile with full width at half maximum (FWHM) of $27.6\pm2.8$ km s$^{-1}$. This is significantly broader than the line spread function of HARPS ($2.7$ km s$^{-1}$). We surmise that the observed broadening could trace the super-rotation in the upper atmosphere of this ultra-hot gas giant.