Chemical tagging in the SDSS-III/APOGEE survey: new identifications of halo stars with globular cluster origins

TL;DR

This study identifies five halo red giant stars with chemical signatures indicating they originated in globular clusters, suggesting a significant fraction of halo stars may have globular cluster origins, based on APOGEE data.

Contribution

The paper introduces a method to identify halo stars with globular cluster origins using chemical tagging in APOGEE data, expanding understanding of halo star formation history.

Findings

2% of halo giants show globular cluster chemical signatures

At least 13% of halo stars may have formed in globular clusters

Chemical tagging reveals comparable numbers of globular cluster migrants and actual cluster stars

Abstract

We present new identifications of five red giant stars in the Galactic halo with chemical abundance patterns that indicate they originally formed in globular clusters. Using data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Survey available through Sloan Digital Sky Survey Data Release 12 (DR12), we first identify likely halo giants, and then search those for the well-known chemical tags associated with globular clusters, specifically enrichment in nitrogen and aluminum. We find that 2% of the halo giants in our sample have this chemical signature, in agreement with previous results. Following the interpretation in our previous work on this topic, this would imply that at least 13% of halo stars originally formed in globular clusters. Recent developments in the theoretical understanding of globular cluster formation raise questions about that interpretation, and we concede the possibility that these migrants represent a small fraction of the halo field. There are roughly as many stars with the chemical tags of globular clusters in the halo field as there are in globular clusters, whether or not they are accompanied by a much larger chemically untaggable population of former globular cluster stars.