Bimodality of light and s-elements in M4 (NGC 6121). A hint for the massive main-sequence star pollution scenario

Context. All globular clusters (GCs) studied in detail host two or more populations of stars (the multiple population phenomenon). Theoretical models suggest that the second population is formed from gas polluted by processed material produced by massive stars of the first generation. However the nature of the polluters is matter of strong debate. Several candidates have been proposed: massive main-sequence stars (fast rotating or binaries), intermediate-mass AGB stars, or SNeII.
Aims: We studied red giant branch (RGB) stars in the GC M4 (NGC 6121) to measure their chemical signature. Our goal is to measure abundances for many key elements (from Li to Eu) in order to derive constraints on the polluters responsible for the multiple populations.
Methods: We observed 23 RGB stars below the RGB-bump using the GIRAFFE spectroscopic facility installed on VLT2. Spectra cover a wide range and allowed us to measure light (Li, C, ¹²C/¹³C, N, O, Na, Mg, Al), α (Si, Ca, Ti), iron-peak (Cr, Fe, Ni), light-s (Y), heavy-s (Ba), and r (Eu) elements. We supplemented this study by analyzing an additional subsample of the UVES spectra in order to gather further clues about light s-elements of different atomic number (Y and Zr).
Results: We confirm the presence of a bimodal population. Stars can be easily separated according to their N content. The two groups have different C, ¹²C/¹³C, N, O, Na content, but share the same Li, C+N+O, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zr, Ba and Eu abundance. Quite surprisingly the two groups differ also in their Y abundance. This result is strongly supported also by the analysis of the UVES spectra.
Conclusions: The absence of a spread in α-elements, Eu and Ba makes SNeII and AGB stars unlikely as polluters. On the other hand, massive main-sequence stars can explain the bimodality of Y through the weak s-process. This statement is confirmed independently also by literature data on Rb and Pb. The lack of a Mg/Al spread and the extension of the [O/Na] distribution suggest that the mass of the polluters is between 20 and 30 M_⊙. This implies a formation time scale for the cluster of 10 ÷ 30 Myr. This result is valid for M4. Other clusters such as NGC 1851, M 22, or ω Cen have different chemical signatures and may require other kinds of polluters.

Based on observations made with ESO telescopes at La Silla Paranal Observatory under program ID 083.B-0083.Table 6 is available in electronic form at http://www.aanda.org