Spectroscopy and Photometry of Multiple Populations along the Asymptotic Giant Branch of NGC 2808 and NGC 6121 (M4)

We present a photometric and spectroscopic study of multiple populations along the asymptotic giant branch (AGB) of the intermediate-metallicity globular clusters (GCs) NGC 2808 and NGC 6121 (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 multiwavelength photometry from the Hubble Space Telescope UV survey of Galactic GCs and ground-based photometry, as well as 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 compositions. In M4, we have identified two main populations of stars with different Na/O content lying on distinct AGBs in the ${m}_{{\rm{F}}438{\rm{W}}}$ versus ${C}_{{\rm{F}}275{\rm{W}},{\rm{F}}336{\rm{W}},{\rm{F}}438{\rm{W}}}$ and the V versus ${C}_{{\rm{U}},{\rm{B}},{\rm{I}}}$ pseudo-color–magnitude diagrams. In the more massive and complex GC NGC 2808, three groups of stars with different chemical abundances occupy different locations on the so-called "chromosome map" photometric diagram constructed for AGB stars. 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 from primordial to high contents of $Y\sim 0.32$. By contrast, from our data set, there is no evidence for stars with extreme helium abundance ($Y\sim 0.38$) on the AGB, suggesting that the most He-rich stars of NGC 2808 do not reach this phase