Discovery of a nitrogen-enhanced mildly metal-poor binary system:
  Possible evidence for pollution from an extinct AGB Star
  We report the serendipitous discovery of a nitrogen-rich, mildly metal-poor
([Fe/H]=-1.08) giant star in a single-lined spectroscopic binary system found
in the SDSS-IV Apache Point Observatory Galactic Evolution Experiment
(APOGEE-2) survey, Data Release 14 (DR14). Previous work has assumed that the
two percent of halo giants with unusual elemental abundances have been
evaporated from globular clusters, but other origins for their abundance
signatures, including binary mass transfer, must also be explored. We present
the results of an abundance re-analysis of the APOGEE-2 high-resolution
near-infrared spectrum of 2M12451043+1217401 with the Brussels Automatic
Stellar Parameter (BACCHUS) automated spectral analysis code, and re-derive
manually the main element families, namely the light elements (C, N), elements
(O, Mg, Si), iron-peak element (Fe), \textit{s}-process element (Ce), and the
light odd-Z element (Al). Our analysis confirm the N-rich nature of
2M12451043+1217401, which has a [N/Fe] ratio of $+0.69$, and shows that the
abundances of C and Al are slightly discrepant from that of a typical mildly
metal-poor RGB star, but exhibit Mg, Si, O and \textit{s}-process abundances
(Ce) of typical field stars. We also detect a particularly large variability in
its radial velocity over the period of the APOGEE-2 observations, and the most
likely orbit fit to the radial velocity data has a period of $730.89\pm106.86$
days, a velocity semi-amplitude of $9.92 \pm 0.14$ km s$^{-1}$, and an
eccentricity of $\sim 0.1276 \pm0.1174$, which support the hypothesis of a
binary companion, and that has probably been polluted by a now-extinct AGB
star.
