The chemical compositions of multiple stellar populations in the globular cluster NGC 2808

TL;DR

This study uses ground-based photometry and spectroscopic data to analyze the chemical compositions of multiple stellar populations in the globular cluster NGC 2808, revealing detailed elemental variations and challenging existing stellar evolution models.

Contribution

It provides the most comprehensive chemical characterization of NGC 2808's stellar populations by combining ground-based photometry with spectroscopic elemental abundances, including AGB stars.

Findings

Light elements vary among populations; iron peak elements are nearly constant.

AGB stars show chemical variations, indicating they are affected by multiple populations.

Detection of an extreme O-poor AGB star challenges current stellar evolution theories.

Abstract

Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U,B,I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a sample of 70 red giant branch (RGB) and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterisation of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another; whereas the iron peak elements show negligible variation between the different populations (at a level of $\lesssim0.10$~dex). Our AGB stars are also characterised by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models which suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manqu\'e star.