The Terminator Region Atmosphere of the hot Jupiter WASP-77Ab with ESPRESSO/VLT observations

TL;DR

This study uses high-resolution ESPRESSO/VLT transmission spectroscopy to analyze the atmosphere of hot Jupiter WASP-77Ab, detecting atmospheric signals and assessing orbital alignment, but cannot definitively confirm planetary atmospheric features.

Contribution

First high-resolution transmission spectra of WASP-77Ab obtained with ESPRESSO, including analysis of spin-orbit alignment and atmospheric signals, highlighting the need for further atmospheric characterization.

Findings

Detected Hα, Hβ, and CaII H with >3.5σ significance

Planet's orbit is nearly aligned with the stellar equator

Partial planetary contribution suggested in spectral lines

Abstract

Atmospheric studies are essential for elucidating the formation history, evolutionary processes, and atmospheric dynamics of exoplanets. High-resolution transmission spectroscopy offers the advantage of detecting subtle variations in stellar spectral profiles, thereby enabling the identification of the sources of observed signals. In this study, we present the transmission spectra of the exoplanet WASP-77Ab, a hot Jupiter with a 1.36-day orbital period around a G8 host star with $V=11.29$ mag. These observations were conducted using the high-resolution spectrograph ESPRESSO at the Very Large Telescope over three transit events. We analyze the Rossiter-McLaughlin effect for WASP-77A and determine a projected spin-orbit angle of ${\lambda = 16.131^{\circ}}^{+2.106}_{-2.324}$, indicating that the planet's orbit is nearly aligned. Following the generation of transmission spectra for the three nights, we model and correct for center-to-limb variation and the Rossiter-McLaughlin effects. In the residual transmission spectra, we detect H$\alpha$, H$\beta$ and CaII H with a significance exceeding 3.5$\sigma$. After applying 0.1-0.5 \r{A} masks to the cores of these lines to mitigate stellar contamination, all them still shows visible absorptions although not significant, suggesting at least partial planet contribution to them. Therefore, we are yet unable to confirm or reject the planetary origin of these spectral signals based on the current data set. Further investigation of WASP-77Ab's atmosphere, particularly in areas beyond the terminator region, is essential to illuminate the planet's two-dimensional atmospheric structure.