Precise measurement of WASP-31 b's Rossiter-McLaughlin effect and characterization of the planet transmission spectra

TL;DR

This study refines the measurement of WASP-31 b's Rossiter-McLaughlin effect and attempts to characterize its transmission spectrum, highlighting the challenges posed by line-distortion contamination in high-resolution spectroscopy.

Contribution

The paper applies advanced correction methods to analyze the RM effect and transmission spectrum of WASP-31 b, emphasizing the impact of POLD contamination on atmospheric detection.

Findings

Confirmed low sky-projected spin-orbit angle of WASP-31 b.

Did not detect atmospheric species due to POLD contamination.

Highlight the need for improved techniques to isolate planetary signals.

Abstract

Context. Hot Jupiters are ideal natural laboratories to investigate atmospheric composition and dynamics. However, high-resolution transmission spectroscopy is currently limited by our capability of removing planet-occulted line-distortion (POLD) contamination from the signal. Aims. In this paper, we aim to characterize the transmission spectrum of WASP-31 b from two and a half transits observed with the ESPRESSO spectrograph at the VLT. Methods. The Rossiter-McLaughlin (RM) signature was analyzed using the RM "revolutions" method. Before extracting the transmission spectrum of the planet, we corrected the dataset for telluric lines using molecfit and further modeled the POLD deformations using EvE. Results. We confirm the planet low sky-projected spin-orbit angle from previous studies and further refine its value to $\lambda = -0.09^{+0.31}_{-0.32}$ deg. We do not detect any species (including previously detected species such as K or CrH) in the planetary atmosphere. In most cases the non-detections are due to the strong POLDs contamination or lack of observable lines in the ESPRESSO wavelength range, and so previous detections cannot be ruled out. Conclusions. Planet-occulted line-distortion contamination continues to be the main limitation of high-resolution transmission spectroscopy for species present in both the star and the planet, hindering atmospheric detections even with state-of-the-art models, in particular for planets with a low sky-projected spin-orbit angle. Developing advanced techniques to isolate planetary signatures is of utmost importance in the advent of ELT-like observations.