Na-O Anticorrelation And HB I. The Na-O anticorrelation in NGC 2808

TL;DR

This study analyzes the Na-O anticorrelation in 120 red giant stars of NGC 2808, revealing multiple stellar populations with varying oxygen and sodium abundances, and discusses implications for cluster formation and horizontal branch morphology.

Contribution

It provides the first detailed Na-O anticorrelation analysis for NGC 2808, identifying multiple stellar populations and their chemical distributions using high-resolution spectroscopy.

Findings

Presence of O-normal, O-poor, and super O-poor stars

Most stars are O-rich, unlike similar clusters

Evidence of a small metallicity spread

Abstract

We derived atmospheric parameters and elemental abundances of Fe, O and Na for about 120 red giant stars in the Galactic globular cluster NGC 2808. Our results are based on the analysis of medium-high resolution (R=22000-24000) GIRAFFE spectra acquired with the FLAMES spectrograph at VLT-UT2 as a part of a project aimed at studying the Na-O anticorrelation as a function of physical parameters in globular clusters. We present here the anticorrelation of Na and O abundances in NGC 2808, and we discuss the distribution function of stars along this relation. Besides a bulk of O-normal stars, with composition typical of field halo stars, NGC 2808 seems to host two other groups of O-poor and super O-poor stars. In this regard, NGC 2808 is similar to M 13, the template cluster for the Na-O anticorrelation. However, at variance with M 13, most stars in NGC 2808 are O-rich. This might be related to the horizontal branch morphologies which are very different in these two clusters. The average metallicity we found for NGC 2808 is [Fe/H]=-1.10 (rms=0.065 dex, from 123 stars). We also found some evidence of a small intrinsic spread in metallicity, but more definitive conclusions are hampered by the presence of a small differential reddening.