Cepheid Metallicity in the Leavitt Law (C--MetaLL) survey: VIII. High-Resolution IGRINS Spectroscopy of 23 Classical Cepheids: Validating NIR Abundances

Abstract

Context. While most chemical abundance studies of Cepheids rely on optical spectroscopy, near-infrared (NIR) observations offer advantages in terms of reduced extinction and access to new elemental tracers. Aims. We aim to validate NIR-based abundance determinations against optical results and to explore the diagnostic power of spectral lines inaccessible in the optical domain. The H and K bands allow us to trace elements such as P, K, and Yb, while also probing obscured Galactic regions and more distant Cepheids. Methods. We obtained high-resolution (R=45000) H- and K-band spectra for 21 Galactic and 2 LMC Classical Cepheids using IGRINS. Atmospheric parameters were derived from photometry and line-depth ratios (Teff), empirical calibrations (log g), and spectral fitting. Abundances of 16 elements were determined via LTE full spectral synthesis and compared with optical literature values. Results. We find excellent agreement between NIR and optical abundances, confirming the reliability of IGRINS-based measurements. The Fe, Mg, and Si gradients match previous optical determinations. We provide the first homogeneous NIR-based measurements of P, K, and Yb in Cepheids, consistent with chemical evolution models. The two LMC Cepheids in our sample, also studied optically, serve as extragalactic benchmarks for validating NIR abundances in low-metallicity regimes. Conclusions. High-resolution NIR spectroscopy yields accurate chemical abundances in Cepheids, consistent with optical results, and grants access to additional nucleosynthetic tracers. These results support future large NIR spectroscopic surveys with instruments such as MOONS, ELT, and JWST for Galactic and extragalactic archaeology.

19 pages, 8 figures

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