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Nearly all Galactic globular clusters host stars that display characteristicabundance anti-correlations, like the O-rich/Na-poor pattern typical of fieldhalo stars, together with O-poor/Na-rich additional components. A recentspectroscopic investigation questioned the presence of O-poor/Na-rich starsamongst a sample of asymptotic giant branch stars in the cluster M 4, atvariance with the spectroscopic detection of a O-poor/Na-rich component alongboth the cluster red giant branch and horizontal branch. This is contrary towhat is expected from the cluster horizontal branch morphology and horizontalbranch stellar evolution models. Here we have investigated this issue byemploying the CUBI= (U-B)-(B-I) index, that previous studies have demonstratedto be very effective in separating multiple populations along both the redgiant and asymptotic giant branch sequences. We confirm previous results thatthe RGB is intrinsically broad in the V-CUBI diagram, with the presence of twocomponents which nicely correspond to the two populations identified byhigh-resolution spectroscopy. We find that AGB stars are distributed over awide range of CUBI values, in close analogy with what is observed for the RGB,demonstrating that the AGB of M4 also hosts multiple stellar populations.

Multiple populations along the asymptotic giant branch of the globular cluster M 4