The double RGB in M 2: C, N, Sr and Ba abundances

TL;DR

This study investigates the chemical differences between the two red giant branch sequences in globular cluster M 2, revealing distinct s-process element and C, N abundances that suggest a complex star formation history.

Contribution

It provides detailed chemical abundance measurements for two RGB populations in M 2, highlighting their differences and supporting the cluster's complex formation history.

Findings

Red RGB stars are richer in C and N than blue RGB stars.

Clear separation in s-process element abundances between the two RGBs.

Both groups show a C-N anti-correlation.

Abstract

The globular cluster M 2 has a photometrically detected double red giant branch (RGB) sequence. We investigate here the chemical differences between the two RGBs in order to gain insight in the star formation history of this cluster. The low-resolution spectra, covering the blue spectral range, were collected with the MODS spectrograph on the LBT, and analyzed via spectrum synthesis technique. The high quality of the spectra allows us to measure C, N, Ba, and Sr abundances relative to iron for 15 RGB stars distributed along the two sequences. We add to the MODS sample C and N measurements for 35 additional stars belonging to the blue RGB sequence, presented in Lardo et al. (2012). We find a clear separation between the two groups of stars in s-process elements as well as C and N content. Both groups display a C-N anti-correlation and the red RGB stars are on average richer in C and N with respect to the blue RGB. Our results reinforce the suggestion that M2 belongs to the family of globular clusters with complex star formation history, together with Omega Cen, NGC 1851, and M 22.