Multiple populations along the asymptotic giant branch of the globular
  cluster M 4
  Nearly all Galactic globular clusters host stars that display characteristic
abundance anti-correlations, like the O-rich/Na-poor pattern typical of field
halo stars, together with O-poor/Na-rich additional components. A recent
spectroscopic investigation questioned the presence of O-poor/Na-rich stars
amongst a sample of asymptotic giant branch stars in the cluster M 4, at
variance with the spectroscopic detection of a O-poor/Na-rich component along
both the cluster red giant branch and horizontal branch. This is contrary to
what is expected from the cluster horizontal branch morphology and horizontal
branch stellar evolution models. Here we have investigated this issue by
employing the CUBI= (U-B)-(B-I) index, that previous studies have demonstrated
to be very effective in separating multiple populations along both the red
giant and asymptotic giant branch sequences. We confirm previous results that
the RGB is intrinsically broad in the V-CUBI diagram, with the presence of two
components which nicely correspond to the two populations identified by
high-resolution spectroscopy. We find that AGB stars are distributed over a
wide 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.
