The GAPS programme at TNG LXVI. A homogeneous search for Na i and its possible variability in ten gas giant exoplanets

TL;DR

This study investigates sodium absorption variability in ten gas giant exoplanets using high-resolution spectroscopy, finding that variability is mainly due to data quality and stellar activity rather than planetary properties.

Contribution

It provides a homogeneous analysis of sodium absorption across ten exoplanets, revealing variability patterns and confirming sodium detection in several cases with improved methodology.

Findings

Sodium absorption detected in 5 out of 10 exoplanets.

Variability in sodium signals linked to data quality and stellar activity.

Confirmed sodium detection in HD 189733 b, KELT-9 b, KELT-20 b, WASP-69 b, and WASP-76 b.

Abstract

The neutral sodium resonance doublet (Na i D) has been detected in the upper atmosphere of several close-in gas giants, through high-resolution transmission spectroscopy. We aim to investigate whether its variability is linked to the planets' properties, the data quality, or the accuracy of the system parameters used. Using the public code SLOPpy, we extracted the transmission spectrum in the Na i D region of ten gas giants for which a large number of HARPS-N observations are available. We modelled the absorption signals found, performing an MCMC analysis, and converted the measured absorption depth to the corresponding atmospheric height over which most sodium absorption occurs. While two targets (GJ 436 b and KELT-7 b) show no Na i D feature, we found variability in the transmission spectrum of the other targets. Three of them (HD 209458 b, WASP-80 b, and WASP-127 b) present absorption on only some nights, while in the other five targets (HD 189733 b, KELT-9 b, KELT-20 b, WASP-69 b, and WASP-76 b), a significant absorption signal is present on most of the nights analysed. Except for WASP-69 b, the measured absorption depths lead to a ratio of the two Na I D depths that is compatible with or slightly larger than one. As was expected from literature, the relative atmospheric height follows an empirical exponential trend as a function of a scaled product of the planet's equilibrium temperature and surface gravity. We confirm the sodium detection on HD 189733 b, KELT-9 b, KELT-20 b, WASP-69 b, and WASP-76 b. The signal detected in WASP-127 b requires further observations for definitive confirmation. We exclude a planetary origin for the signals found on HD 209458 b and WASP-80 b. The sodium absorption variability does not appear to be related to planetary properties, but rather to data quality, sub-optimal data treatment, or stellar activity.