# Photometry and spectroscopy of multiple populations along the AGB of NGC2808 and NGC6121 (M4)

## TL;DR

This study combines photometry and spectroscopy to analyze multiple stellar populations along the AGB in two globular clusters, revealing diverse chemical compositions and helium abundance variations, with implications for stellar evolution.

## Contribution

It provides the first detailed chemical and photometric analysis of AGB populations in NGC2808 and NGC6121, highlighting differences in helium content and population distribution.

## Key findings

- Multiple populations with different chemical compositions are present on the AGB.
- In NGC2808, AGB hosts populations with helium from primordial to Y~0.32.
- No evidence of extreme helium abundance stars (Y~0.38) on the AGB.

## Abstract

We present a photometric and spectroscopic study of multiple populations along the asymptotic-giant branch (AGB) of the intermediate-metallicity globular clusters (GCs) NGC2808 and NGC6121 (M4). Chemical abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Co, Ni, Zn, Y, and Ce in AGB stars from high-resolution FLAMES+UVES@VLT spectra are reported for both clusters. Our spectroscopic results have been combined with multi-wavelength photometry from the HST UV survey of Galactic GCs and ground-based photometry, plus proper motions derived by combining stellar positions from ground-based images and Gaia DR1. Our analysis reveals that the AGBs of both clusters host multiple populations with different chemical composition. In M4 we have identified two main populations of stars with different Na/O content, lying on distinct AGBs in the mF438W vs. C_F275W,F336W,F438W and the V vs.C_U,B,I pseudo-CMDs. In the more massive and complex GC NGC2808 three groups of stars with different chemical abundances occupy different locations on the so-called "chromosome map" photometric diagram. The spectroscopic+photometric comparison of stellar populations along the AGB and the red giants of this GC suggests that the AGB hosts stellar populations with a range in helium abundances spanning from primordial up to high contents of Y~0.32. On the other hand, from our dataset, there is no evidence for stars with extreme helium abundance (Y~0.38) on the AGB, suggesting that the most He-rich stars of NGC2808 do not reach this phase.