Gaia FGK Benchmark Stars: Selecting Infrared Lines for Abundance Determination

Abstract

The advent of new and more powerful infrared spectrographs has significantly motivated the advancement of the study of atomic and molecular line lists and stellar atmosphere models. While optical abundance determinations rely on extensively validated line lists and modeling frameworks, infrared measurements still face larger uncertainties, largely driven by the choice of atmospheric models and the quality of the available atomic data. In this work, we aim to deliver a homogeneous and reproducible set of atomic absorption lines in the Y, J, and H bands (9800 - 18000 (Angstrom)), based exclusively on laboratory atomic data. We analyse CRIRES spectra of six Gaia FGK Benchmark Stars spanning a wide range in effective temperature, surface gravity, and chemical composition. Synthetic spectra are computed using the benchmark stellar parameters, and each transition is evaluated independently in every star through a quantitative sequence that examines line depth, saturation, blending (purity), and the agreement between observed and synthetic line profiles. We identify a set of robust atomic transitions in these bands that remain consistent across the full range of stellar parameters represented in our sample. Lines of alpha-elements such as Mg I, Si I, and Ca I, together with several Fe I transitions, satisfy all robustness criteria. Among the neutron-capture species explored, only Sr II provides lines that consistently meet our requirements. Beyond the specific list of accepted transitions, this study demonstrates that a fully quantitative, multi-criteria framework provides a transparent and reproducible foundation for near-infrared line validation as laboratory data, stellar atmosphere models, and instrumentation continue to improve.

Figures (7)

• Figure 1: Spectral cuts for each of the three bands are shown for the representative stars, after telluric correction, RV correction, and normalization. The stars are sorted by temperature, making clear how the line profiles evolve from warmer to cooler GBS.
• Figure 2: Methodology flow chart where the flags for each process are defined.
• Figure 3: Synthesis of the Si i absorption line for $\epsilon$ Eri (K Dwarf) in the Y band. The line in crimson indicates the synthesis for Si i only.
• Figure 4: Number of absorption lines with $d > 0.03$. For both M Giant and F Dwarf.
• Figure 5: Example of a non-saturated line, the absorption feature of the Sr ii line at 10036.65 Å in a M Giant star. Top panel: Synthetic spectra computed for different Sr ii abundances, ranging from $[{Sr\,\textsc{ii}/H}]=-2.0$ to $+2.0$ dex. Bottom panel: Corresponding curve of growth (COG). Each point represents a synthetic spectrum at a given abundance offset. The red dot marks the reference abundance used for the stellar model, i.e., the point where the local COG slope is evaluated to diagnose whether the line is saturated. The dashed vertical line indicates the corresponding $\log(Nf\lambda)$ value.