The double red giant branch in M2: C, N, Sr and Ba abundances

The globular cluster M2 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 Multi-Object Double CCD Spectrograph (MODS) on the Large Binocular Telescope, and analysed 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. 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 anticorrelation 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 ω Cen, NGC 1851 and M22.