Barium Surface Abundances of Blue Stragglers in the Open Cluster NGC 6819

TL;DR

This study investigates barium surface abundances in blue straggler and main-sequence stars in NGC 6819, revealing some blue stragglers with significant barium enhancement likely due to mass transfer, challenging simple binary formation models.

Contribution

It provides new barium abundance measurements for stars in NGC 6819 and identifies blue stragglers with enrichment suggesting complex formation histories.

Findings

Five blue stragglers show significant barium enhancement.

Most blue stragglers are consistent with main-sequence star abundances.

Some barium-enhanced blue stragglers lack radial-velocity evidence of companions.

Abstract

We present the barium surface abundance of 12 blue stragglers (BSs) and 18 main-sequence (MS) stars in the intermediate-age open cluster NGC 6819 (2.5 Gyr) based on spectra obtained from the Hydra Multi-object Spectrograph on the WIYN 3.5 m telescope. For the MS stars we find [Fe/H] = $+$0.05 $\pm$ 0.04 and [Ba/Fe] = $-$0.01 $\pm$ 0.10. The majority of the BS stars are consistent with these values. We identify five BSs with significant barium enhancement. These stars most likely formed through mass transfer from an asymptotic giant branch star that polluted the surface of the BS with the nucleosynthesis products generated during thermal pulsations. This conclusion aligns with the results from the substantial work done on the BSs in old open cluster NGC 188 that identifies mass transfer as the dominant mechanism for BS formation in that open cluster. However, four of the BSs with enhanced barium show no radial-velocity evidence for a companion. The one star that is in a binary is a double-lined system, meaning the companion is not a white dwarf and not the remnant of a prior AGB star. In this paper we attempt to develop a consistent scenario to explain the origin of these five BSs.