About Metallicity Variations in the Local Galactic Interstellar Medium

TL;DR

This paper reviews recent measurements of metallicity variations in the local interstellar medium, comparing different tracers, and concludes that the chemical composition is largely homogeneous, challenging the idea of widespread low-metallicity clouds.

Contribution

It provides a comprehensive comparison of metallicity data from various tracers and discusses the implications for the homogeneity of the local interstellar medium.

Findings

Metallicity variations are consistent among HII regions, B-type stars, Cepheids, and young clusters.

Neutral clouds show larger metallicity variations than other tracers.

The hypothesis of many low-metallicity clouds is incompatible with theoretical models and observations.

Abstract

In this paper we discuss and confront recent results on metallicity variations in the local interstellar medium, obtained from observations of HII regions and neutral clouds of the Galactic thin disk, and compare them with recent high-quality metallicity determinations of other tracers of the chemical composition of the interstellar medium as B-type stars, classical Cepheids and young clusters. We find that the metallicity variations obtained for these last kinds of objects are consistent with each other and with that obtained for HII regions but significantly smaller than those obtained for neutral clouds. We also discuss the presence of a large population of low-metallicity clouds as the possible origin for large metallicity variations in the local Galactic thin disk. We find that such hypothesis does not seem compatible with: (a) what is predicted by theoretical studies of gas mixing in galactic disks, and (b) the models and observations on the metallicity of high-velocity clouds and its evolution as they mix with the surrounding medium in their fall onto the Galactic plane. We conclude that that most of the evidence favors that the chemical composition of the interstellar medium in the solar neighborhood is highly homogeneous.