(G. Canocchi+, 2026) ================================================================================ 3D NLTE Sodium abundances in late-type stars. Abundance corrections and synthetic spectra. G. Canocchi, E. X. Wang, A. M. Amarsi, K. Lind, M. Racca ================================================================================ Abstract: Context. Neutral sodium is an important tracer of the Galactic chemical evolution, a powerful diagnostic of different stellar populations, and the subject of detailed studies of exoplanet atmospheres via transmission spectroscopy. Aims. This work aims to study and quantify the errors in stellar analyses of Na I lines caused by the use of one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE). Methods. We studied the line formation of nine Na I lines in FGK dwarfs and giants via, for the first time, 3D~non-LTE (NLTE) radiative transfer post-processing with the code Balder on 3D radiation hydrodynamic stellar atmospheres from the Stagger grid spanning Teff= 4000 to 6500 K, log g = 1.5 to 5.0, and [Fe/H]=-4 to +0.5. Results. We find that the 3D NLTE abundance corrections relative to 1D LTE tend to be negative, and more positive than the corresponding 1D NLTE corrections. This reflects more efficient overionisation in the steeper temperature gradient of the 3D models. The corrections are typically less severe than -0.1 dex for weak lines, but become much larger for saturated lines in low-gravity giants (log g < 2.0), even reaching -0.7 dex. However, for the D resonance lines, the 3D NLTE corrections relative to 1D LTE become slightly positive at the lowest metallicities in our grid, typically around +0.05 dex at [Fe/H]=-4. Conclusions. We make our 3D NLTE grid, together with interpolation routines based on radial basis functions and fully connected feedforward neural networks, publicly available. This will enable more accurate determination of sodium abundances in present and forthcoming stellar spectroscopic surveys, particularly for metal-poor stars, as well as a better characterisation of the Na I D lines in exoplanet atmospheres. Description: Grid of synthetic stellar spectra for the most important spectral lines of Na I in 3D NLTE and 3D LTE in FGK-type stars (see Fig. 1 or Table A.1 in this paper). The file 3Dflux_all.fits contains the normalized fluxes in 3D NLTE and 3D LTE at the nodes of the grid. The folders rbf/ and ffnn/ contain the interpolation models for the line profiles and the reduced equivalent widths, respectively. All the synthetic spectra were produced with the public 3D Stagger-model atmospheres (Rodriguez-Diaz et al. 2024), performing radiative transfer calculations with the Balder code (Amarsi et al. 2018), as described in detail in Sect. 2 of this paper. We also provide the file interp_tools.py with python functions to access, read and interpolate the data to user-defined stellar parameters of Teff, logg, [Fe/H], A(Na) (as well as vmic for 1D models). The interpolate_rbf.py file also contain a script to perform the interpolation of the line profiles with the radial basis functions (RBF) method. Examples are provided in the file examples.py. The synthesized Na I spectral lines are at the following wavelengths: 4982.81 Å, 5684.22 Å, 5689.78 Å, 5891.58 Å, 5897.56 Å, 6154.23 Å, 6160.75 Å, 8182.56 Å, 8197.06 Å, 10749.30 Å. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . this file 3Dflux_all.fits 43 564 grid of synthetic normalized fluxes -------------------------------------------------------------------------------- Byte-by-byte Description of file: 3Dflux.fits -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 6 F6.1 K Teff [4000.0/6500.0] effective temperature 8-10 F3.1 cm/s² logg [1.5/5.0] surface gravity 12-15 F4.1 dex fe_h [-4.0/0.5] metallicity 17-20 F4.1 dex A_Na [1.72/7.72] sodium abundance 22-36 F14.8 nm wavelength [496.93/1075.94] wavelength 38-48 F3.0 --- flux_NLTE [0.01/1.0] 3D NLTE normalized flux 50-59 F3.0 --- flux_LTE [0.03/1.0] 3D LTE normalized flux -------------------------------------------------------------------------------- See also: https://github.com/GloriaCanocchi/SAND Acknowledgements: Gloria Canocchi, gloria.canocchi(at)gmail.com References: None ================================================================================ (prepared by author / pyreadme )