Mapping radial abundance gradients with Gaia-ESO open clusters

Evidence of recent gas accretion in the Milky Way disk

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy
⁵ Dipartimento di Fisica, Sezione di Astronomia, Università degli studi di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
⁶ INFN – Sezione di Trieste, Via A. Valerio 2, 34100 Trieste, Italy
⁷ Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio av. 3, 10257 Vilnius, Lithuania
⁸ Institut für Kernphysik, Technische Universität Darmstadt, Schlossgartenstr. 2, Darmstadt 64289, Germany

^★ Corresponding author; marco.palla@inaf.it

Received: 5 July 2024
Accepted: 7 September 2024

Abstract

Context. Recent evidence from spectroscopic surveys points towards the presence of a metal-poor, young stellar population in the low- α, chemically thin disk. In this context, the investigation of the spatial distribution and time evolution of precise, unbiased abundances is fundamental to disentangle the scenarios of formation and evolution of the Galaxy.

Aims. We study the evolution of abundance gradients in the Milky Way by taking advantage of a large sample of open star clusters, which are among the best tracers for this purpose. In particular, we used data from the last release of the Gaia-ESO survey.

Methods. We performed a careful selection of open cluster member stars, excluding those members that may be affected by biases in spectral analysis. We compared the cleaned open cluster sample with detailed chemical evolution models for the Milky Way, using well-tested stellar yields and prescription for radial migration. We tested different scenarios of Galaxy evolution to explain the data, namely, the two-infall and the three-infall frameworks, which suggest the chemical thin disk is formed by one or two subsequent gas accretion episodes, respectively.

Results. With the performed selection in cluster member stars, we still find a metallicity decrease between intermediate-age (1 < Age/Gyr < 3) and young (Age < 1 Gyr) open clusters. This decrease cannot be explained in the context of the two-infall scenario, even by accounting for the effect of migration and yield prescriptions. The three-infall framework, with its late gas accretion in the last 3 Gyr, is able to explain the low metallic content in young clusters. However, we have invoked a milder metal dilution for this gas infall episode relative to previous findings.

Conclusions. To explain the observed low metallic content in young clusters, we propose that a late gas accretion episode triggering a metal dilution would have taken place, extending the framework of the three-infall model for the first time to the entire Galactic disk.