Unexpected radial trend of the iron abundance in a sample of monometallic Galactic globular clusters

TL;DR

This study reveals an unexpected radial trend of iron abundance in certain Galactic globular clusters, with iron-poor stars more centrally concentrated, challenging previous assumptions about their chemical homogeneity.

Contribution

It provides the first evidence of a radial iron abundance trend in multiple globular clusters using archival data, highlighting complex internal chemical distributions.

Findings

Iron-poor RGB stars are more centrally concentrated in some GCs.

Significant radial iron abundance differences are observed in 47 Tuc, NGC 1851, NGC 3201, and NGC 6752.

The iron abundance trend correlates with features like the RGB bump and horizontal branch fading.

Abstract

We study the relationship between the iron abundance (IA) in red giant branch (RGB) stars and their radial distribution (RD) in Galactic globular clusters (GCs). We relied on publicly available archival data on IA in red giants (RGs) of GCs. We built a sample of ten target GCs in which the number of these RGs exceeded one hundred stars. In each GC of the sample, we compared the RDs of two sub-samples of stars, more iron-rich (IR) and more iron-poor (IP) than the clusters' mean values of [Fe/H]. Their RDs turned out to be different at statistically significant confidence levels in NGC 104 (47 Tuc), NGC 1851, NGC 3201, and NGC 6752 in the sense that the IP RGs were more centrally concentrated than their IR counterparts. In 47 Tuc, the difference is significant at a higher confidence level within the PRAD of R = 8.0', where the IA increases by Delta[Fe/H] ~ 0.03 dex toward the cluster outskirts. In the latter three GCs, Delta[Fe/H] ~ 0.05 dex. Interestingly, the V magnitude of the RGB bump and the horizontal branch was recently shown to fade outward in 47 Tuc and was suggested to originate from a He abundance trend. The fading caused by the IA trend is similar to that observed for the RGB bump. Although the difference between the RDs of IP and IR RGs is statistically insignificant in other GCs, NGC 288 is the only GC of the sample, in which IR RGB stars are formally more centrally concentrated. Interestingly, three of the four GCs are highly concentrated. (Abridged)