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

TL;DR

This study confirms that the asymptotic giant branch of globular cluster M 4 contains multiple stellar populations, using the CUBI index to distinguish between different abundance patterns, aligning with findings on the red giant branch.

Contribution

The paper demonstrates that the AGB in M 4 hosts multiple populations, resolving previous conflicting results and employing the CUBI index for effective population separation.

Findings

AGB stars in M 4 show a wide range of CUBI values.

Multiple populations are present along the AGB, similar to the RGB.

The CUBI index effectively distinguishes different stellar populations.

Abstract

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.