We present the abundance analysis for a sample of 17 red giant branch stars in the metal-poor globular cluster M28 based on high resolution spectra .
This is the first extensive spectroscopic study of this cluster .
We derive abundances of O , Na , Mg , Al , Si , Ca , Ti , V , Cr , Mn , Fe , Co , Ni , Cu , Zn , Y , Zr , Ba , La , Ce , and Eu .
We find a metallicity of [ Fe/H ] =-1.29 \pm 0.01 and an \alpha -enhancement of +0.34 \pm 0.01 ( errors on the mean ) , typical of Halo Globular Clusters in this metallicity regime .
A large spread is observed in the abundances of light elements O , Na , and Al .
Mg also shows an anticorrelation with Al with a significance of 3 \sigma .
The cluster shows a Na-O anticorrelation and a Na-Al correlation .
This correlation is not linear but “ segmented ” and that the stars are not distributed continuously , but form at least 3 well separated sub-populations .
In this aspect M28 resembles NGC 2808 that was found to host at least 5 sub-populations .
The presence of a Mg-Al anticorrelation favor massive AGB stars as the main polluters responsible for the multiple-population phenomenon .