Cepheid Metallicity in the Leavitt Law (C–MetaLL) survey

Context. Classical Cepheids are fundamental primary distance indicators and crucial tracers of the young stellar population in the Milky Way and nearby galaxies. While most chemical abundance studies of Cepheids have been carried out in the optical domain, near-infrared (NIR) spectroscopy offers unique advantages in terms of reduced extinction and access to new elemental tracers. Aims. Our goal is to validate NIR abundance determinations against well-established optical results and to explore the diagnostic power of previously unexplored NIR lines. NIR spectroscopy is far less hampered by interstellar extinction than optical observations, which allows us to probe Cepheids at larger distances and in highly obscured regions of the Galaxy. Moreover, the H and K bands provide access to diagnostic lines of elements (e.g., P, K, and Yb) that are not available in the optical domain. Methods. We acquired high-resolution (R ≈ 45 000) spectra of 21 Galactic and 2 Large Magellanic Cloud (LMC) classical Cepheids with the high-resolution Immersion Grating Infrared Spectrometer (IGRINS) in the H and K bands. Effective temperatures were derived from a photometric approach and line-depth ratios, and the gravities and microturbulent velocities were estimated using empirical calibrations and statistical constraints. The abundances of 16 elements were determined through a full spectral synthesis in local thermodynamic equilibrium. We performed an extensive error analysis and compared our results with previous optical studies of the same stars. Results. Our NIR abundances and the optical literature values agree very well (ΔFe/H ≤ 0.02 dex and σ ≈ 0.07 dex), which confirms the reliability of IGRINS-based measurements. The derived abundance gradients in the Galactic disk are fully consistent with previous optical determinations, with slopes of −0.06, −0.05, and −0.05 dex kpc−1 for Fe, Mg, and Si, respectively. We provide homogeneous determinations of P, K, and Yb abundances from NIR lines for classical Cepheids for the first time, and we report trends that are consistent with Galactic chemical evolution models. Moreover, the two LMC Cepheids included in our sample that were previously analyzed in the optical provide a direct benchmark that confirms the accuracy of NIR abundance determinations in extragalactic metal-poor environments. Conclusions. Our study demonstrates that high-resolution NIR spectroscopy of Cepheids yields robust abundances that are fully compatible with optical results and provides access to additional elements of nucleosynthetic interest. These results pave the way for future large-scale NIR surveys of Cepheids with facilities such as MOONS, ELT, and JWST, which are crucial for tracing the chemical evolution of the Milky Way and nearby galaxies in heavily obscured regions.