High-resolution atmospheric retrievals of WASP-121b transmission spectroscopy with ESPRESSO: Consistent relative abundance constraints across multiple epochs and instruments

Abstract

Recent progress in high-resolution transmission spectroscopy has offered new avenues in which to characterise the atmospheres of transiting exoplanets. High-resolution cross-correlation spectroscopy allows for the unambiguous detection of molecules/atoms. It has also been used to map both atmospheric dynamics and longitudinal variations in the abundance of species across the morning and evening limbs. We present multiple VLT/ESPRESSO observations of the ultra-hot Jupiter WASP-121b, from which we constrain relative abundances of various neutral metals consistently across all observations, whilst accounting for the distortion of the exoplanet’s signal caused by traditional data processing techniques. We also constrain planetary orbital velocities and T-P profiles. We compare our abundance constraints with previous constraints using VLT/UVES transmission spectroscopy of WASP-121b, and find our results to be consistent between observations, and also in agreement with stellar values for species previously detected in the atmosphere of WASP-121b. Our retrieval framework can also be used to identify potential exospheric species, resulting in extended absorption features beyond the transit equivalent Roche limit of WASP-121b (ReqRL ∼ 1.3 Rp). Hα, Fe II, and Ca II were found to extend to high altitudes (1.54 ± 0.04 Rp, 1.17 ± 0.01 Rp, and 2.52 ± 0.34 Rp, respectively), which are broadly consistent with literature values. The consistency of our constraints across multiple high-resolution observations is a strong validation of our model filtering and retrieval framework, as well as the stability of the atmosphere over the timescales of months/years, and could allow for planet formation processes to be inferred from future ground-based observations of exoplanetary atmospheres.

Retrieval Survey of Metals in Six Ultrahot Jupiters: Trends in Chemistry, Rain-out, Ionization, and Atmospheric Dynamics

“…They were also able to constrain Na, with an abundance slightly above that of our upper bound. For WASP-121 b, Maguire et al (2023) retrieved upper limits to Ti similar to ours, but did constrain Na at abundances similar to but slightly above our upper bounds. Furthermore, Gibson et al (2022) constrained Fe, Mg, Cr, and V in WASP-121 b with UVES/VLT observations, which are also in good agreement with our abundance constraints.…”

“…We show the expected volume mixing ratio as well as the ratio of each of species to the total atom content, X tot , of the atmosphere. Typically, high-resolution ground-based observations in the optical probe pressures near ∼0.1 mbar (e.g., Maguire et al 2023).…”

“…Hence, Ti-bearing species may be sequestered deep in the interior of the planet through cold-trapping (e.g., Spiegel et al 2009). Recent work has also shown no evidence for Ti or TiO in WASP-121 b (Hoeijmakers et al 2022;Maguire et al 2023), or TiO in MASCARA-2 b (Johnson et al 2023).…”

“…Determination of the Ca+ abundance will provide a key indicator to the overall ionization of neutral Ca and the total Ca content of the atmosphere, as many of these planets have shown very strong Ca+ absorption (Merritt et al 2021;Kesseli et al 2022;Zhang et al 2022;Pelletier et al 2023). However, including this into retrievals may be challenging as they probe the exosphere (Maguire et al 2023).…”

The hot Neptune WASP-166 b with ESPRESSO III: A blue-shifted tentative water signal constrains the presence of clouds

“…These distortions need to be accounted for to derive accurate and precise estimates of the planetary transmission spectra (see e.g. Brogi et al 2016;Yan et al 2017;Hoeijmakers et al 2020;Casasayas-Barris et al 2021;Seidel et al 2022;Maguire et al 2022, for more details on such effects and strategies to account and correct for them).…”