Classical Cepheids (CCs) are solid distance indicators and tracers of young stellar populations. Dating back to almost one century ago, they have been safely adopted to trace the rotation, the kinematics and the chemical enrichment history of the Galactic thin disk. The main aim of this investigation is to provide iron, oxygen, and sulfur abundances for the largest and most homogeneous sample of Galactic CCs ever analyzed (1118 spectra of 356 objects). The current sample, containing 77 CCs for which spectroscopic metal abundances are provided for the first time, covers a wide range in Galactocentric distances, pulsation modes, and pulsation periods. Optical, high-resolution, and high S/N spectra collected with different spectrographs were adopted to provide homogeneous estimates of the atmospheric parameters (effective temperature, surface gravity, microturbulent velocity) required for abundance determination. Individual distances are based either on trigonometric parallaxes by Gaia DR3 or on distances based near-infrared Period-Luminosity relations. We found that iron and alpha-element radial gradients based on CCs display a well-defined change in the slope for Galactocentric distances larger than ~12kpc. We also found that logarithmic regressions take account for the variation of [X/H] abundances when moving from the inner to the outer disk. Radial gradients for the same elements, but based on open clusters covering a wide range in cluster ages, display similar trends. This means that the flattening in the outer disk is an intrinsic feature of the radial gradients, since it is independent of age. Empirical evidence indicates that the S radial gradient is steeper than the Fe radial gradient. The difference in the slope is a factor of two in the linear fit (-0.081 vs. -0.041dex/kpc) and changes from -1.62 to -0.91 in the logarithmic distance. Moreover, we found that S (explosive nucleosynthesis) is, on average, under-abundant when compared with O (hydrostatic nucleosynthesis). The difference becomes clearer in the metal-poor regime and in dealing with the [O/Fe] and [S/Fe] abundance ratios. We performed a detailed comparison with Galactic chemical evolution models and we found that a constant Star Formation Efficiency for Galactocentric distances larger than 12kpc takes account for the flattening observed in both iron and alpha-elements. To further constrain the impact that predicted S yields for massive stars have on radial gradients, we adopted a "toy model" and we found that the flattening in the outermost regions requires a decrease of a factor of four in the current S predictions. CCs are solid beacons to trace the recent chemical enrichment of young stellar populations. Sulfur photospheric abundances, when compared with other alpha-elements, have the key advantage of being a volatile element. Therefore, stellar S abundances can be directly compared with nebular sulfur abundances in external galaxies.
Cone search capability for table J/A+A/678/A195/table1 (Pulsation parameters, literature Fe abundance, and Heliocentric and Galactocentric distances)
