  Large star-to-star variations of the abundances of proton-capture elements,
such as Na and O, in globular clusters (GCs) are interpreted as the effect of
internal pollution resulting from the presence of multiple stellar populations.
To better constrain this scenario we investigate the abundance distribution of
the heavy element rubidium (Rb) in NGC 6752, NGC 1904, and NGC 104 (47 Tuc).
Combining the results from our sample with those in the literature, we found
that Rb exhibits no star-to-star variations, regardless the cluster
metallicity, with the possible intriguing, though very uncertain, exception of
the metal-rich bulge cluster NGC 6388. If no star-to-star variations will be
confirmed for all GCs, it implies that the stellar source of the proton-capture
element variations must not have produced significant amounts of Rb. This
element is observed to be enhanced at extremely high levels in
intermediate-mass AGB (IM-AGB) stars in the Magellanic Clouds (i.e., at a
metallicity similar to 47 Tuc and NGC 6388). This may present a challenge to
this popular candidate polluter, unless the mass range of the observed IM-AGB
stars does not participate in the formation of the second-generation stars in
GCs. A number of possible solutions are available to resolve this conundrum,
also given that the Magellanic Clouds observations are very uncertain and may
need to be revised. The fast rotating massive stars scenario would not face
this potential problem as the slow mechanical winds of these stars during their
main-sequence phase do not carry any Rb enhancements; however, these candidates
face even bigger issues such as the production of Li and the close
over-imposition with core-collapse supernova timescales. Observations of Sr,
Rb, and Zr in metal-rich clusters such as NGC 6388 and NGC 6441 are sorely
needed to clarify the situation.
