DREAM

III. A helium survey in exoplanets on the edge of the hot Neptune desert with GIANO-B at TNG

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: gloria.guilluy@edu.unito.it
² Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290, Versoix, Switzerland
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ Département de Physique, Institut Trottier de Recherche sur les Exoplanètes, Université de Montréal, 1375 Ave. Thérèse-Lavoie-Roux, Montréal, QC H2V 0B3, Canada
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi, 46, 23807 Merate (LC), Italy

Received: 15 March 2023
Accepted: 3 July 2023

Abstract

Context. The population of close-in exoplanets features a desert of hot Neptunes whose origin remains uncertain. These planets may have lost their atmosphere, eroding into mini-Neptunes and rocky super-Earths below the desert. Direct observations of evaporating atmospheres are essential to derive mass-loss estimates and constrain this scenario. The metastable He I triplet at 1083.3 nm represents a powerful diagnostic of atmospheric evaporation because it traces the hot gas in extended exoplanet atmospheres while being observed from the ground. In addition, it is located at the bright near-infrared stellar continuum and is very weakly affected by interstellar medium (ISM) absorption.

Aims. We carried out a homogeneous He I transmission spectroscopy survey, targeting a selected sample of nine planets along the different edges of the desert, to interpret the absorption line profile with evaporation models and to better understand the role of photoevaporation in the desert formation.

Methods. We observed one transit per planet using the high-resolution, near-infrared spectrograph GIANO-B mounted on the Telescopio Nazionale Galileo telescope. We focused our analysis on the He I triplet, based on a comparison of the in-transit and out-of-transit observations, and we computed high-resolution transmission spectra. We then employed the 1D p-winds model to calculate the planetary thermospheric structures and to interpret the observed transmission spectra.

Results. We found no signatures of planetary absorption in the He I triplet in any of the investigated targets. We thus provided 3 σ upper-limit estimations on the thermosphere absorption, temperature and mass loss, and combined them with past measurements to search for correlations with parameters such as the stellar mass and XUV flux, which are thought to be key drivers in the formation of the He I triplet.

Conclusions. These results strengthen the importance of performing homogeneous surveys and analyses in bringing clarity to He I detections and (thereby) to plausible Neptunian desert origins. Our findings corroborate literature expectations that state the He I absorption signal is correlated with the stellar mass and the received XUV flux. However, when translated in terms of mass-loss rates, these trends seem to disappear. Thus, further studies are essential to shed light on this aspect and to better understand the photoevaporation process.