Na-O Anticorrelation and HB. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218

TL;DR

This study used high-resolution spectroscopy to identify two distinct stellar populations with different helium contents in the globular cluster NGC 6218, based on Na-O anticorrelation and luminosity variations.

Contribution

First extensive spectroscopic analysis revealing He-rich and He-poor populations in NGC 6218 through Na-O anticorrelation and luminosity differences.

Findings

NGC 6218 has a homogeneous metallicity of [Fe/H]=-1.31.

Large Na-O variations indicate early formation of anticorrelation.

Detection of two populations with different He content based on RGB bump luminosity.

Abstract

We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope UT2 to derive atmospheric parameters, metallicities and abundances of O and Na for 79 red giant stars in the Galactic globular cluster NGC 6218 (M 12). We analyzed stars in the magnitude range from about 1 mag below the bump to the tip of the Red Giant Branch. The average metallicity we derive is [Fe/H]=-1.31+/-0.004+/-0.028 dex (random and systematic errors, respectively), with a very small star-to-star scatter (rms=0.033 dex), from moderately high-resolution Giraffe spectra. This is the first extensive spectroscopic abundance analysis in this cluster. Our results indicate that NGC 6218 is very homogeneous as far as heavy elements are concerned. On the other hand, light elements involved in the well known proton-capture reactions of H-burning at high temperature, such as O and Na, show large variations, anticorrelated with each other, at all luminosities along the red giant branch. The conclusion is that the Na-O anticorrelation must be established in early times at the cluster formation. We interpret the variation of Na found near the RGB-bump as the effect of two distinct populations having different bump luminosities, as predicted for different He content. To our knowledge, NGC 6218 is the first GC where such a signature has been spectroscopically detected, when combined with consistent and homogeneous data obtained for NGC 6752 to gain in statistical significance.