Ba stars and other binaries in first and second generation stars in globular clusters

TL;DR

This study measured barium content in over 1200 stars across 15 globular clusters, finding no variation in Ba abundance among populations and a lower binary fraction in second-generation stars, suggesting denser formation environments.

Contribution

It provides the first large-scale Ba abundance analysis in GC stars and links binary fractions to formation environments of different stellar generations.

Findings

No variation in [Ba/Fe] ratios among stellar populations within clusters.

First-generation stars have a Ba-star fraction similar to field stars (~2%).

Second-generation stars show a significantly lower binary fraction (~1%).

Abstract

The determination of the Ba abundance in globular cluster (GC) stars is a very powerful test to address several issues in the framework of multiple population scenarios. We measured the Ba content for a sample of more than 1200 stars in 15 Galactic GCs, using high-resolution FLAMES/Giraffe spectra. We found no variation in [Ba/Fe] ratios for different stellar populations within each cluster; this means that low-mass asymptotic giant branch stars do not significantly contribute to the intra-cluster pollution. Very interestingly, we obtained that the fraction of Ba-stars in first generation (FG) stars is close to the values derived for field stars ($\sim$2\%); on the other hand, second generation (SG) stars present a significant lower fraction. An independent and successful test, based on radial velocity variations among giant stars in NGC~6121, confirms our finding: the binary fraction among FG stars is about $\sim$12\% to be compared with $\sim$1\% of SG stars. This is an evidence that SG stars formed in a denser environment, where infant mortality of binary systems was particularly efficient.