Aluminium abundances in five discrete stellar populations of the globular cluster NGC 2808^⋆,⋆⋆

¹ INAF-Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
e-mail: eugenio.carretta@oabo.inaf.it
² INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 20 November 2017
Accepted: 25 January 2018

Abstract

We observed a sample of 90 red giant branch (RGB) stars in NGC 2808 using FLAMES/GIRAFFE and the high resolution grating with the set-up HR21. These stars have previous accurate atmospheric parameters and abundances of light elements. We derived aluminium abundances for them from the strong doublet Al i 8772–8773 Å as in previous works of our group. In addition, we were able to estimate the relative CN abundances for 89 of the stars from the strength of a large number of CN features. When adding self-consistent abundances from previous UVES spectra analysed by our team, we gathered [Al/Fe] ratios for a total of 108 RGB stars in NGC 2808. The full dataset of proton-capture elements is used to explore in detail the five spectroscopically detected discrete components in this globular cluster. We found that various classes of polluters are required to reproduce (anti)-correlations among all proton-capture elements in the populations P2, I1, and I2 with intermediate composition. This is in agreement with the detection of lithium in lower RGB second generation stars, requiring at least two kind of polluters. For chemically homogeneous populations, the best subdivision of our sample is into six components as derived from statistical cluster analysis. By comparing different diagrams [element/Fe] versus [element/Fe], we show for the first time that a simple dilution model is not able to reproduce all the subpopulations in this cluster. Polluters of different masses are required. NGC 2808 is confirmed to be a tough challenge to any scenario for globular cluster formation.