The metallicity variations along the chromosome maps: The Globular Cluster 47 Tucanae

Abstract

The "chromosome maps" (ChMs) of globular clusters (GCs) have revealed that these ancient structures are not homogeneous in metallicity in various ways, and in different natures. The Type II GCs generally display larger variations, sometimes coupled with slow neutron capture (s) element enrichment on the ChMs redder sequences, which has been interpreted as due to multiple generations of stars. On the other hand, most GCs have inhomogeneous first populations (1P) in the form of large ranges in the Delta(F275W,F814W) values, pointing towards a not fully mixed pristine molecular cloud. We analyse the chemical composition the GC 47 Tucanae, which shows both inhomogeneous 1P stars and, although not formally a Type II GC, hosts a small number of stars distributed on a red side of the main stream of ChM stars. Our results suggest that 1P stars are not homogeneous in the overall metallicity, with variations of the order of ~0.10 dex in all the chemical species. The anomalous stars distributed on a redder sequence of the ChM, are further enriched in metals, but without any evidence for a significant enrichment in the s elements. Our three second population stars located on the normal component of the map, have metallicities similar to those of the metal-richer 1P group, suggesting that this population formed from these stars. Although three stars is a too-small sample to draw strong conclusions, the low spread in metals of these objects might point towards a formation in a fully mixed medium, possibly after a cooling flow phase.