All Globular Clusters (GCs) studied in detail so far host two or more populations of stars. Theoretical models suggest that the second population is formed from gas polluted by processed material produced by massive stars of the first generation. However the nature of the polluter is a matter of strong debate. Several candidates have been proposed: massive main-sequence stars (fast rotating or binaries), intermediate-mass AGB stars, or SNeII. We studied red giant branch (RGB) stars in the GC M4 (NGC 6121) to measure their chemical signature. We confirm the presence of a bimodal population, first discovered by Marino et al. (2008). The two groups have different C,$^{12}$C/$^{13}$C,N,O,Na content, but share the same Li,C+N+O,Mg,Al,Si,Ca,Ti,Cr,Fe,Ni,Zr,Ba and Eu abundance. 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 AGB stars unlikely as polluters. On the other hand, massive main-sequence stars can explain the bimodality of Y through the weak s-process. This stement is confirmed independently also by literature data on Rb and Pb. Observations suggest that the mass of the polluters is between 20 and 30 M$_{odot}$. This implies a formation time scale for the cluster of 10$div$30 Myrs. This result is valid for M4. Other clusters like NGC 1851, M22, or $omega$ Cen have different chemical signatures and may require other kinds of polluter.
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