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We present the barium surface abundance of 12 blue stragglers (BSs) and 18main-sequence (MS) stars in the intermediate-age open cluster NGC 6819 (2.5Gyr) based on spectra obtained from the Hydra Multi-object Spectrograph on theWIYN 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 withthese values. We identify five BSs with significant barium enhancement. Thesestars most likely formed through mass transfer from an asymptotic giant branchstar that polluted the surface of the BS with the nucleosynthesis productsgenerated during thermal pulsations. This conclusion aligns with the resultsfrom the substantial work done on the BSs in old open cluster NGC 188 thatidentifies mass transfer as the dominant mechanism for BS formation in thatopen cluster. However, four of the BSs with enhanced barium show noradial-velocity evidence for a companion. The one star that is in a binary is adouble-lined system, meaning the companion is not a white dwarf and not theremnant of a prior AGB star. In this paper we attempt to develop a consistentscenario to explain the origin of these five BSs.

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