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