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

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.

💡 Research Summary

The paper presents the eighth installment of the Cepheid Metallicity in the Leavitt Law (C‑MetaLL) survey, focusing on high‑resolution near‑infrared (NIR) spectroscopy of 23 classical Cepheids (21 Galactic, 2 Large Magellanic Cloud) obtained with the Immersion Grating Infrared Spectrometer (IGRINS) on Gemini South. The instrument delivers simultaneous H‑ and K‑band coverage (1.45–2.45 µm) at a resolving power of R≈45 000, allowing the authors to acquire spectra with signal‑to‑noise ratios between 80 and 160 per pixel.

The primary goals are (1) to validate chemical abundances derived from NIR spectra against the extensive body of optical Cepheid abundance work, and (2) to exploit NIR‑only diagnostic lines—particularly those of phosphorus (P I), potassium (K I), and ytterbium (Yb II)—that are inaccessible in the optical.

Effective temperatures (T_eff) are determined by two independent methods: (a) a photometric approach using Gaia BP/RP colors calibrated against established Cepheid colour‑temperature relations, and (b) a spectroscopic line‑depth‑ratio (LDR) technique employing carefully selected Ti I/Fe I and V I line pairs in the H and K bands. The two temperature scales agree within –90 ± 200 K, with typical uncertainties of 50–300 K. Surface gravities (log g) and microturbulent velocities (ξ) cannot be constrained directly in the NIR because Fe II lines are absent and most lines are weakly saturated. Instead, the authors adopt an empirical log g relation that depends on T_eff and pulsation period, and a statistical ξ value (ξ = 3.3 ± 0.6 km s⁻¹) derived from the full C‑MetaLL sample of over 500 Cepheids. These indirect estimates are justified by the excellent agreement of the final abundances with optical literature.

Chemical abundances for 16 elements (Fe, Mg, Si, S, Ca, Ti, Cr, Ni, Y, La, Ce, Nd, P, K, Yb, and others) are obtained via LTE full‑spectral synthesis. Model atmospheres are computed with ATLAS9, and synthetic spectra are generated with SYNTHE using line lists primarily from Castelli & Hubrig (2004). The authors perform a thorough error analysis, propagating uncertainties in T_eff, log g, ξ, and S/N through Monte‑Carlo simulations.

Key results include:

• Fe consistency: The mean offset between NIR and optical