3D non-LTE Ca II line formation in metal-poor FGK stars

I. Abundance corrections, radial velocity corrections, and synthetic spectra

¹ Department of Astronomy, Stockholm University, Albanova University Center, Roslagstullsbacken 21, 106 91 Stockholm, Sweden
² Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden

^★ Corresponding author; cis.lagae@astro.su.se

Received: 4 November 2024
Accepted: 13 March 2025

Abstract

Context. The Ca II near-ultraviolet resonance doublet (H&K) and the near-infrared triplet (CaT) are among the strongest features in stellar spectra of FGK-type stars. These spectral lines remain prominent down to extremely low metallicities and are thus useful for providing stellar parameters via ionisation balance, for Galactic chemical evolution, and as radial velocity diagnostics. However, the majority of studies that model these lines in late-type stars still rely on simplified one-dimensional (1D) hydrostatic model atmospheres and the assumption of local thermodynamic equilibrium (LTE).

Aims. We present 3D non-LTE radiative transfer calculations of the CaT and H&K lines in an extended grid of 3D model atmospheres of a metal-poor FGK type. We investigated the impact of 3D non-LTE effects on abundances, line bisectors, and radial velocities.

Methods. We used a subset of 3D model atmospheres from the recently published STAGGER-grid to synthesise spectra in 3D (non-)LTE with Balder for nine different calcium-to-iron ratios. For comparison, similar calculations were performed in 1D (non-)LTE using models from the MARCS grid.

Results. Abundance corrections for the CaT lines relative to 1D LTE range from +0.1≳Δ_1L^3N≳−1.0 dex, with more severe corrections for strong lines in giants. With fixed line strength, the abundance corrections become more negative with increasing effective temperature and decreasing surface gravity. Radial velocity corrections relative to 1D LTE based on cross-correlation of the whole line profile range from −0.2 kms⁻¹ to +1.5 kms⁻¹, with more severe corrections where the CaT lines are strongest. The corrections are even more severe if the line core alone is used to infer the radial velocity.

Conclusions. The line strengths and shapes, and consequently the abundance and radial velocity corrections, are strongly affected by the chosen radiative transfer assumption, 1/3D (non)-LTE. We release grids of theoretical spectra that can be used to improve the accuracy of stellar spectroscopic analyses based on the Ca II triplet lines.