  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.
