Oxygen, sulfur, and iron radial abundance gradients of classical Cepheids across the Galactic thin disk

TL;DR

This study analyzes the radial abundance gradients of iron, oxygen, and sulfur in classical Cepheids across the Galactic thin disk, revealing a slope change beyond 12 kpc and implications for Galactic chemical evolution models.

Contribution

It provides the largest homogeneous dataset of elemental abundances in Galactic Cepheids, highlighting the flattening of gradients and the distinct behavior of sulfur compared to other alpha-elements.

Findings

Radial gradients show a slope change beyond 12 kpc.

Sulfur has a steeper gradient than iron.

Sulfur is under-abundant relative to oxygen on average.

Abstract

Classical Cepheids (CCs) are solid distance indicators and tracers of young stellar populations. Our aim is to provide iron, oxygen, and sulfur abundances for the largest and most homogeneous sample of Galactic CCs ever analyzed. The current sample covers a wide range in Galactocentric distances (RG), pulsation modes and periods. High-resolution and high S/N spectra collected with different spectrographs were adopted to estimate the atmospheric parameters. Individual distances are based on Gaia trigonometric parallaxes or on near-infrared Period-Luminosity relations. We found that Fe and alpha-element radial gradients based on CCs display a well-defined change in the slope for RG larger than 12 kpc. Radial gradients based on open clusters, covering a wide range in age, display similar trends, meaning 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 radial gradient for S is steeper than for Fe. The difference in the slope is a factor of two in the linear fit. We also found that S is, on average, under-abundant compared with O. We performed a detailed comparison with Galactic chemical evolution models and we found that a constant Star Formation Efficiency for RG larger than 12 kpc takes account for the flattening in both Fe 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. Sulfur photospheric abundances, compared with other alpha-elements, have the key advantage of being a volatile element. Therefore, stellar S abundances can be directly compared with nebular S abundances in external galaxies.