A Tale of Sodium: From Stars to Exoplanets with 3D non-LTE spectroscopy

by Gloria Canocchi (Stockholm University)

Friday 12 Sept 2025, 10:30 → 11:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

Neutral sodium (Na I) is a key tracer of Galactic chemical evolution in late-type stars and the most commonly detected species in exoplanet atmospheres through transmission spectroscopy. Classical hydrostatic one-dimensional (1D) local thermodynamic equilibrium (LTE) models, however, can overestimate abundances—by up to 0.5 dex in giant stars—because the LTE assumption breaks down. In high-resolution transmission spectroscopy of exoplanets, additional corrections for the center-to-limb variation (CLV) of stellar lines and the Rossiter–McLaughlin (RM) effect during transits are crucial, as their combination can mimic or erase planetary absorption. Such effects cannot be modeled accurately with 1D LTE atmospheres, which also fail to reproduce spatially resolved observations of the solar disk. More realistic three-dimensional (3D) radiation-hydrodynamic models and non-local thermodynamic equilibrium (non-LTE) line formation are therefore required for an accurate treatment of the CLV effect.

To address these challenges, we computed a state-of-the-art grid of 3D non-LTE synthetic spectra for FGK-type stars using 3D radiation-hydrodynamic atmospheres from the new, extended, and refined Stagger-grid (Rodriguez-Diaz et al. 2024). This grid enables more accurate sodium abundance determinations in current and future large stellar surveys (e.g., GALAH, 4MOST) and in exoplanet observations with upcoming high-resolution instruments such as ANDES at the ELT.

In this talk, I will present applications of these 3D non-LTE models from my PhD work: from spatially resolved solar data with the Swedish 1-m Solar Telescope, to giant exoplanet atmospheres observed with ESPRESSO, to stellar populations from GALAH DR4.