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

TL;DR

This study validates high-resolution near-infrared abundance measurements of Cepheids by comparing NIR (H and K bands) IGRINS spectra to optical results for 23 Cepheids (21 Galactic, 2 LMC). Teff, log g, and microturbulence are determined with a combination of photometry, line-depth ratios, and empirical calibrations, and 16 elements are derived via LTE full spectral synthesis. The authors find excellent agreement with optical abundances and reproduce known Galactic abundance gradients, while delivering the first homogeneous NIR measurements of P, K, and Yb in Cepheids, including extragalactic benchmarks from LMC stars. These results demonstrate the reliability and diagnostic power of NIR spectroscopy for Cepheid chemistry and support future large-scale NIR surveys with facilities like MOONS, ELT, and JWST to map the chemical evolution of the Milky Way and nearby galaxies in obscured regions.

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.

Figures (8)

• Figure 1: Upper panel: comparison between effective temperatures derived from spectroscopy and photometry, with the red solid line representing the bisecting line. Bottom panel: Residuals, computed as $\Delta T_{\text{eff}}$ = $T_{eff}^{Phot}- T_{eff}^{Spec}$, show a mean offset of $-90 \pm 200$ K. With a solid red line we represent null difference ($\Delta T_{\text{eff}} = 0$), while with a blue dashed line the mean residual ($-90$ K). We also show, limited by blue dotted lines, the region of $\pm 1 \sigma$ scatter.
• Figure 2: Distribution of microturbulence velocities for the sample of 538 classical Cepheids from the C-MetaLL survey. The Gaussian fit to the distribution yields $\xi$ = 3.3 $\pm$ 0.6 km s$^{-1}$, as labelled.
• Figure 3: Representative sub-sample of 13 observed spectra (black) in the region between $\lambda$ = 22450 and 22750 Å. The best-fitting synthetic spectra are overplotted in red. The spectra are ordered, from top to bottom, by decreasing effective temperature (see Table \ref{['tab:param']}. The main spectral lines have been identified at the top and highlighted with black dotted vertical lines.
• Figure 4: Radial abundance gradients of selected elements in the Galactic disc. Red points represent the galactic cepheids abundances derived in this work from IGRINS H–K spectra, while grey points correspond to optical measurements from the C‑MetaLL sample. Solid red lines indicate the linear fits adopted from Trentin2024.
• Figure 5: Chemical abundances of elements (in their [X/Fe] form) plotted against iron abundance [Fe/H]. The horizontal dashed line at [X/Fe] = 0 indicates the solar abundance ratio. Meaning of the colors is as in Fig. \ref{['gradient']}.