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

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 $λ= -0.09^{+0.31}_{-0.32}$ 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.

Figures (21)

• Figure 1: Location of WASP-31b (red point) in radius versus insolation flux diagram of known exoplanets (small black points), zoomed-in on the hot-Jupiter region. Extracted from NASA Exoplanet archive on 24 March, 2025
• Figure 2: Radial-velocity observations of ESPRESSO data. The Rossiter-McLaughlin anomaly is visible, showing the typical signature of an aligned planet as previously deduced by brown2012 using a single HARPS transit. Nights are color-coded separately in blue (night #1), orange (#2), and green (#3).
• Figure 3: Observed simultaneous photometric light curves (top panels) from ECAM for the three ESPRESSO nights. The best-fit models from CONAN for each night are color-coded separately in blue (night #1), orange (#2), and green (#3). The residuals for each night are shown in the bottom panel with an offset. The horizontal line is the continuum of 0.
• Figure 4: Intrinsic CCF per night (left panels) and their respective residuals (right panels). The RM signature is well visible during the transit as a bright track. The dashed horizontal lines correspond to T14 contact points (orange) and T23 (green). The solid red lines (with black outlines) correspond to spectra that were excluded from the analysis. The dotted black lines correspond to the expected local stellar velocity using the best-fit model. The first night's residuals show a systematic trend, likely due to line asymmetry.
• Figure 5: Final posterior corner plot of e revolutions analysis of RM anomaly. All values are well-defined, with an anticorrelation between the contrast and FWHM of the line profile. The contours show 1$\sigma$, a region containing 50% of accepted steps and 2$\sigma$ levels, as defined by the corner package. The solid lines correspond to the median value of the posterior distribution function, which is used as the measured value.