The Metal Contents of Milky Way Globular Clusters

Abstract

ABSTRACTGlobular clusters in the Milky Way contain total masses of heavy elements ranging from a few solar masses to as great as 104 M⊙. Given that a massive star of greater than 40 M⊙ can produce in excess of 12 M⊙ of heavy elements, it is possible that at least some globular clusters contain the metals from only a very small number of supernovae, although many clusters contain a sufficiently large mass of metals that the contribution from 10 to 200 or more supernovae is required. More than one supernova event would be needed within an initially metal‐free cloud to raise the heavy‐element abundance to a level greater than [A/H] = -1.8 dex. Many halo field stars have metallicities much lower than this. It is possible that such stars formed from cold primeval clouds with masses in excess of 2 × 106M⊙ that were self‐enriched by only one or two supernovae.The metallicity distribution of Milky Way globular clusters shows some features that may vary with cluster mass. By binning clusters into integrated‐magnitude groups of MV,t less than −8.0, −8.0 to −6.5, and greater than −6.5, it is shown that the percentage of clusters with metallicities [A/H] > -1.0 increases with decreasing cluster mass. In addition to this trend, the occurrence of intermediate metallicities with [A/H] = -1.0 ± 0.2 is mainly restricted to globular clusters fainter than MV,t = -8.0. It is possible that some of these intermediate‐metallicity clusters formed during an era in the Galactic halo that favored the production of lower mass systems.