WIYN Open Cluster Study. XC. Barium Surface Abundances of Blue Straggler Stars in the Open Clusters NGC 7789 and M67

TL;DR

This study examines barium surface abundances in blue straggler stars across several open clusters to understand their formation via AGB mass transfer, revealing correlations with cluster age and binary properties.

Contribution

It provides new insights into the link between AGB donor mass, mass transfer efficiency, and binary characteristics in BSS formation in open clusters.

Findings

43% of BSSs are Ba-enriched across studied clusters.

Ba enrichment decreases with increasing cluster age.

Long-period binaries are common among Ba-enriched BSSs.

Abstract

We investigate barium (Ba) abundances in blue straggler stars (BSSs) in two open clusters, NGC 7789 (1.6 Gyr) and M67 (4 Gyr), as signatures of asymptotic-giant-branch (AGB) mass transfer. We combine our findings with previous Ba abundance analyses in NGC 6819 (2.5 Gyr) and NGC 188 (7 Gyr). Out of 35 BSSs studied in NGC 7789, NGC 6819, and M67, 15 (43$\pm$11%) are Ba-enriched; no BSSs in NGC 188 are Ba-enriched. The Ba abundances of enriched BSSs show an anticorrelation with cluster age, ranging from an enrichment of [Ba/Fe]$\sim$+1.5 dex in NGC 7789 to [Ba/Fe]$\sim$+1.0 dex in M67. The Ba-enriched BSSs all lie in the same region of the HR diagram, irrespective of cluster age or distance from the main-sequence turnoff. Our data suggest a link between AGB donor mass and mass-transfer efficiency in BSSs, in that less massive AGB donors tend to undergo more conservative mass transfer. We find that 40$\pm$16% of the Ba-enriched BSSs are in longer-period spectroscopic binaries with orbital periods less than 5000 days. Those Ba-enriched BSSs that do not exhibit radial-velocity variability suggest AGB mass-transfer in wide binaries by either wind mass transfer or wind Roche-lobe overflow. Given the preponderance of long orbital periods in the BSSs of M67 and NGC 188 and the frequency of Ba enrichment in NGC 7789, NGC 6819, and M67, it may be that AGB mass transfer is the dominant mechanism of BSS formation in open clusters older than 1 Gyr.