Three discrete groups with homogeneous chemistry along the red giant branch in the globular cluster NGC 2808

TL;DR

This study reveals three distinct stellar groups with homogeneous chemistry in NGC 2808, linking chemical composition to photometric sequences and suggesting multiple polluters influenced the cluster's formation.

Contribution

First detailed chemical analysis linking discrete stellar populations with photometric sequences in NGC 2808, indicating multiple polluters affected its chemical evolution.

Findings

Stars cluster into three chemically distinct groups.

Clear correspondence between photometric sequences and chemical groups.

Different polluters likely contributed to the cluster's chemical diversity.

Abstract

We present the homogeneous reanalysis of Mg and Al abundances from high resolution UVES/FLAMES spectra for 31 red giants in the globular cluster NGC 2808. We found a well defined Mg-Al anticorrelation reaching a regime of subsolar Mg abundance ratios, with a spread of about 1.4 dex in [Al/Fe]. The main result from the improved statistics of our sample is that the distribution of stars is not continuous along the anticorrelation as they are neatly clustered into three distinct clumps each with different chemical composition. One group (P) shows the primordial composition of field stars of similar metallicity, and the other two (I and E) have increasing abundances of Al and decreasing abundances of Mg. The fraction of stars we found in the three components (P: 68%, I: 19%, E: 13%) is in excellent agreement with the number ratios computed for the three distinct main sequences in NGC 2808: for the first time there is a clear correspondence between discrete photometric sequences of dwarfs and distinct groups of giants with homogeneous chemistry. The composition of the I group cannot be reproduced by mixing of matter with extreme processing in hot H-burning and gas with pristine, unprocessed composition, as also found in the recent analysis of three discrete groups in NGC 6752. This finding suggests that different classes of polluters were probably at work also in NGC 2808.