Building the Galactic halo from globular clusters: evidence from chemically unusual red giants

TL;DR

This study identifies stars in the Galactic halo with chemical signatures indicating they originated from globular clusters, suggesting these clusters contributed significantly to the halo's formation.

Contribution

It provides observational evidence that a substantial fraction of the Galactic halo stars originated from globular clusters, based on chemical abundance patterns.

Findings

16 stars with globular cluster-like chemical signatures identified

At least 17% of the stellar halo mass originated from globular clusters

Inner halo contains a higher proportion of these chemically peculiar stars

Abstract

We present a spectroscopic search for halo field stars that originally formed in globular clusters. Using moderate-resolution SDSS-III/SEGUE-2 spectra of 561 red giants with typical halo metallicities (-1.8 < [Fe/H] < -1.0), we identify 16 stars, 3% of the sample, with CN and CH bandstrength behavior indicating depleted carbon and enhanced nitrogen abundances relative to the rest of the data set. Since globular clusters are the only environment known in which stars form with this pattern of atypical light-element abundances, we claim that these stars are second-generation globular cluster stars that have been lost to the halo field via normal cluster mass-loss processes. Extrapolating from theoretical models of two-generation globular cluster formation, this result suggests that globular clusters contributed significant numbers of stars to the construction of the Galactic halo: we calculate that a minimum of 17% of the present-day mass of the stellar halo was originally formed in globular clusters. The ratio of CN-strong to CN-normal stars drops with Galactocentric distance, suggesting that the inner-halo population may be the primary repository of these stars.