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 \pm 0.004 \pm 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 .