Origin of Abundance Inhomogeneity in Globular Clusters

We numerically investigate abundance properties of the Galactic globular clusters (GCs) by adopting a new “external pollution” scenario. In this framework, GCs are assumed to originate in forming low-mass dwarfs embedded in dark matter subhalos at very high redshifts (z) and thus be chemically influenced by field AGB stars of the dwarfs during early GC formation processes. GCs within a dwarf galaxy therefore can be formed from the mixture of (i) gas ejected from the field AGB stars formed earlier in the dwarf and (ii) the interstellar gas infalling to the central region of the dwarf. In this external pollution scenario, the ratio of the total mass of infalling gas to that of AGB ejecta during GC formation in a dwarf (s) and the time scale of gas infall (σI) are the most important key parameters that can determine abundance properties of GCs. We mainly investigate the abundance inhomogeneity among light elements (e.g., C, N, O, Na, and Al) of stars in GCs by using the latest stellar yield models of metal-poor AGB stars with and without third dredge-up. Our principal results for the models with no third dredge-up, which are more consistent with observations, are as follows. (1) Both [N/Fe] and [C/Fe] can be diverse among stars within a GC owing to chemical pollution from field AGB stars. [N/Fe] distributions in some GCs can clearly show bimodality whereas [C/Fe] is monomodal in most models. [N/Fe] distributions depend on s such that models with smaller s (i.e., larger mass fraction of AGB ejecta used for GC formation) show the [N/Fe] bimodality more clearly. (2) N-rich, C-poor stars in GCs also have higher He abundances owing to pollution from massive AGB stars with He-rich ejecta. The number fraction of He-rich stars (Y > 0.30) is higher for the models with smaller s and shorter σI for 3 ≤ s ≤ 24 and 10 ≤ σI ≤ 10 7 yr. He abundances of stars correlate with [N/Fe] and [Al/Fe] and anticorrelate with [C/Fe], [O/Fe], and [Na/Fe] within GCs in our models. (3) Although our model can much better explain the observed C-N and Mg-Al anticorrelations than previous theoretical models, it is in strong disagreement with the observed O-Na anticorrelation. (4) This model naturally provides an explanation for the large fraction of CNstrong stars without recourse to an implausible IMF. Based on these results for the above external pollution scenario, we discuss the long-standing problem of the CN-bimodality prevalent in the Galactic GCs, the possible helium abundance inhomogeneity in these systems, and their horizontal branch morphologies.