Cepheid metallicities from low-resolution near-infrared spectra: validation against the optical reference scale

Classical Cepheid metallicities trace recent chemical enrichment and help assess systematics in Cepheid-based distance measurements. They are derived mainly from high-resolution optical spectroscopy, which provides the benchmark abundance scale but limits measurements to nearby systems. Extending such studies to more distant Cepheids with extremely large telescopes requires complementary low-resolution near-infrared methods and careful treatment of model--data residuals. We test whether low-resolution SpeX/IRTF Y+J-band spectra can yield metallicities compatible with homogenised optical abundances. We analyse 14 Galactic classical Cepheids with periods of approximately 13--69 d, supplemented by two shorter-period stars from the IRTF Spectral Library. Using Bayesian full-spectrum fitting based on MARCS/TURBOSPECTRUM spectra and a regularised covariance likelihood, we infer [M/H] and compare it with homogenised optical [Fe/H]. For the primary 14-star sample, the inferred metallicities reproduce the optical reference scale with a mean offset of-0.05 dex, a scatter of 0.09 dex, and a robust scatter of 0.12 dex; the median posterior uncertainty is 0.22 dex. Including the two additional stars changes these values only slightly. Compared with a diagonal likelihood, the covariance-aware treatment reduces the mean offset and improves the stability of the uncertainty calibration. Low-resolution Y+J spectra therefore contain usable metallicity information for classical Cepheids when short-range correlated residuals and localised modelling imperfections are treated explicitly. The resulting near-infrared metallicity scale provides a practical foundation for extending Cepheid metallicity work to larger and more distant samples.