APOGEE discovery of a chemically atypical star disrupted from NGC 6723 and captured by the Milky Way bulge

Abstract

The central (‘bulge’) region of the Milky Way is teeming with a significant fraction of mildly metal-deficient stars with atmospheres that are strongly enriched in cyanogen (12C14N). Some of these objects, which are also known as nitrogen-enhanced stars, are hypothesised to be relics of the ancient assembly history of the Milky Way. Although the chemical similarity of nitrogen-enhanced stars to the unique chemical patterns observed in globular clusters has been observed, a direct connection between field stars and globular clusters has not yet been proven. In this work, we report on high-resolution, near-infrared spectroscopic observations of the bulge globular cluster NGC 6723, and the serendipitous discovery of a star, 2M18594405−3651518, located outside the cluster (near the tidal radius) but moving on a similar orbit, providing the first clear piece of evidence of a star that was very likely once a cluster member and has recently been ejected. Its nitrogen abundance ratio ([N/Fe] ≳ + 0.94) is well above the typical Galactic field-star levels, and it exhibits noticeable enrichment in the heavy s-process elements (Ce, Nd, and Yb), along with moderate carbon enrichment; all characteristics are known examples in globular clusters. This result suggests that some of the nitrogen-enhanced stars in the bulge likely originated from the tidal disruption of globular clusters.

APOGEE Detection of N-rich stars in the tidal tails of Palomar 5

“…This discovery is significant in providing unequivocal confirmation that N-rich stars born in globular clusters are lost to the field, and thus providing a bridge between both field and GC enriched populations. Recent evidence to this association has been found by Fernández-Trincado et al (2021a), who identified a chemically peculiar star just beyond the tidal radius of the bulge cluster NGC 6723. However, the optical photometry, stellar parameters, and high [C/Fe] of the star in question suggest that it may in fact be an N-normal AGB star.…”

The Milky Way Bulge Extra-tidal Star Survey: BH 261 (AL 3)

“…In particular, the metal-poor stars in the field of the bulge appear to be connected to inner Galaxy GCs. Field stars with [N/Fe] overabundances are thought to be former members of a population of GCs that was previously dissolved and/or evaporated (e.g., Schiavon et al 2017;Fernández-Trincado et al 2021). This is the same mechanism that has been shown to donate stars to the halo (e.g., Martell et al 2011;Koch et al 2019).…”