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All Globular Clusters (GCs) studied in detail so far host two or morepopulations of stars. Theoretical models suggest that the second population isformed from gas polluted by processed material produced by massive stars of thefirst generation. However the nature of the polluter is a matter of strongdebate. Several candidates have been proposed: massive main-sequence stars(fast rotating or binaries), intermediate-mass AGB stars, or SNeII. We studiedred giant branch (RGB) stars in the GC M4 (NGC 6121) to measure their chemicalsignature. We confirm the presence of a bimodal population, first discovered byMarino et al. (2008). The two groups have different C,$^{12}$C/$^{13}$C,N,O,Nacontent, but share the same Li,C+N+O,Mg,Al,Si,Ca,Ti,Cr,Fe,Ni,Zr,Ba and Euabundance. Quite surprisingly the two groups differ also in their Y abundance.The absence of a spread in $\alpha$-elements, Eu and Ba makes SNeII and AGBstars unlikely as polluters. On the other hand, massive main-sequence stars canexplain the bimodality of Y through the weak s-process. This stement isconfirmed independently also by literature data on Rb and Pb. Observationssuggest that the mass of the polluters is between 20 and 30 M$_{\odot}$. Thisimplies a formation time scale for the cluster of 10$\div$30 Myrs. This resultis valid for M4. Other clusters like NGC 1851, M22, or $\omega$ Cen havedifferent chemical signatures and may require other kinds of polluter.

Bimodality of light and s-elements in M4 (NGC 6121)