Cepheid Metallicity in the Leavitt Law (C-MetaLL) survey: X First spectroscopic characterization of Galactic Type II Cepheids and calibration of the Period-Wesenheit-Metallicity relations

Abstract

Type II Cepheids (T2Cs) are Population II pulsating stars that serve as distance indicators through their period--luminosity relation. Along with RR Lyrae stars and the tip of the red giant branch, they offer an independent route to the Hubble constant, helping assess systematic uncertainties in its determination. We aim to investigate the impact of chemical composition on T2Cs' period--luminosity relations by building a homogeneous spectroscopic sample of Galactic T2Cs with iron and oxygen abundances. We observed 80 Galactic T2Cs with UVES at the ESO Very Large Telescope. For 74 stars, we derived stellar parameters and iron and oxygen abundances through a self-consistent spectroscopic analysis based on equivalent widths of absorption lines. This sample nearly doubles the number of T2Cs with spectroscopic metallicity measurements available in the literature. We used these abundances to test theoretical predictions for the instability strip and, together with $Gaia$ parallaxes, to derive Period--Wesenheit--metallicity (PWZ) relations. In agreement with theoretical predictions, the metallicity coefficient ($γ$) is small and statistically consistent with zero in all relations, ranging from approximately $-0.02$ to $+0.17$ mag dex$^{-1}$. We also estimated distances to 22 Galactic Globular Clusters hosting T2Cs, finding agreement within uncertainties with independent literature estimates. Our results show that metallicity has, at most, a minor impact on T2Cs' pulsation properties and distance scale, supporting their reliability as Population II distance indicators.