On the self-enrichment scenario of galactic globular clusters: Constraints on the IMF

Aims. Galactic globular cluster (GC) stars exhibit abundance patterns which are not shared by their field counterparts, e.g. the well documented O-Na and Mg-Al anticorrelations. A widely held hypothesis is that the gas was “polluted” by stars more massive than the presently observed low-mass stars. In the framework of this “self-enrichment” scenario for GCs, we present a new method to derive the Initial Mass Function (IMF) of the polluters, by using the O/Na abundance distribution. Methods. We focus on NGC 2808, a GC for which the largest sample of O and Na abundance determinations is presently available. We use the abundance distribution of [O/Na] to derive the amount of polluted material with respect to that of original composition. We explore in details two scenarios for the self-enrichment of the cluster, which differ by the assumptions made on the composition of the polluter ejecta. In each case we consider two classes of possible “culprits” : massive Asymptotic Giant Branch (AGB) stars (4-9 M⊙) and winds of massive stars (WMS) in the mass range 10-100 M⊙. Results. We obtain upper limits for the slope of the IMF (assumed to be given by a power-law) of the stars initially more massive than the present turnoff mass. We also derive lower limits for the amount of stellar residues in NGC 2808. Conclusions. We find that the polluter IMF had to be much flatter than presently observed IMFs in stellar clusters; this is in agreement with the results of two other methods for GC IMF determination, which we also discuss. Additionaly, we find that the present mass of the GC should be totally dominated by stellar remnants if the polluters were AGB stars; this is not the case if the culprits are WMS. We critically analyse the advantages and shortcomings of each potential polluter class, and we find the WMS scenario more attractive.