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Globular clusters display significant variations in their light-elementcontent, pointing to the existence of a second stellar generation formed fromthe ejecta of an earlier generation. The nature of these internal polluters isstill a matter of debate: the two most popular scenarios indicateintermediate-mass AGB stars and fast rotating massive stars. Abundancesdetermination for some key elements can help distinguish between thesecompetitor candidates. We present in this paper Y abundances for a sample of103 red giant branch stars in NGC 6121. Within measurement errors, we find thatthe [Y/Fe] is constant in this cluster contrary to a recent suggestion. For asubsample of six stars we also find [Rb/Fe] to be constant, consistent withprevious studies showing no variation in other s-process elements. We alsopresent a new set of stellar yields for intermediate-mass AGB stellar models of5 and 6 solar masses, including heavy element s-process abundances. Theuncertainties on the mass-loss rate, the mixing-length parameter, and thenuclear reaction rates have a major impact on the stellar abundances. Withinthe IM-AGB pollution scenario, the constant abundance of heavy elements insidethe cluster requires a marginal s-process efficiency in IM-AGB stars. Such aconstrain could still be satisfied by the present models assuming a strongermass-loss rate. The uncertainties mentioned above are limiting the predictivepower of intermediate-mass AGB models. For these reasons, at the moment we arenot able to clearly rule out their role as main polluters of the secondpopulation stars in globular clusters.

On the internal pollution mechanisms in the globular cluster NGC 6121 (M4): heavy-element abundances and AGB models