Discovery of a nitrogen-enhanced metal-poor binary system: Possible evidence for pollution from an extinct massive AGB Star

Abstract

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. Here, we verify the large N overabundance of 2M12451043+1217401 and re-determined the chemical abundances for a wider range of chemical species, including light and heavy elements. Our analysis confirms the N-rich nature of 2M12451043+1217401, which has a [N/Fe] ratio of $+0.69$, and shows that the abundances of Al and Mg are mildly discrepant from that of a typical metal-poor RGB star, but exhibit C, Si, O and \\textit{s}-process abundances of typical field stars. We detect a particularly large variability in its radial velocity (RV) 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. The present analysis shows a remarkable agreement between the observations and the theoretical yields from a likely massive (5--7 M$_{\\odot}$) asymptotic giant branch (AGB) star, which supports the idea that this star has been polluted in the past from an already extinct AGB companion, that is now possibly a white dwarf.