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

Abstract

Aims: 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). Methods: 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. They span a wide range of projected radial distance (PRAD) in their parent GCs. Results: 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, where the IA increases by δ[Fe/H] ∼ 0.03 dex toward the cluster outskirts. In the latter three GCs, δ[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. We estimated the fading caused by the IA trend. It 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. We checked whether the trend could be caused by a possible spurious effect, in particular due to systematically brighter IP than IR RGs. NGC 3201 was the only GC where difference between the RDs of IP and IR RGs became insignificant after corrections were applied. The latest data on the IA in a sample of RGs in NGC 3201 confirmed that IP RGs are clearly more centrally concentrated. However, a spurious nature of the trend cannot be fully ruled out. Our results imply that the unusual radial trend of IA in GCs, if real, may occur fairly frequently in GCs with an internally small scatter of the [Fe/H] ratio. Interestingly, three of the four GCs are highly concentrated. © 2013 ESO.