Enrichment by supernovae in globular clusters with multiple populations
  The most massive globular cluster in the Milky Way, omega Centauri, is
thought to be the remaining core of a disrupted dwarf galaxy, as expected
within the model of hierarchical merging. It contains several stellar
populations having different heavy elemental abundances supplied by supernovae
-- a process known as metal enrichment. Although M22 appears to be similar to
omega Cen, other peculiar globular clusters do not. Therefore omega Cen and M22
are viewed as exceptional, and the presence of chemical inhomogeneities in
other clusters is seen as `pollution' from the intermediate-mass
asymptotic-giant-branch stars expected in normal globular clusters. Here we
report Ca abundances for seven globular clusters and compare them to omega Cen.
Calcium and other heavy elements can only be supplied through numerous
supernovae explosions of massive stars in these stellar systems, but the
gravitational potentials of the present-day clusters cannot preserve most of
the ejecta from such explosions. We conclude that these globular clusters, like
omega Cen, are most probably the relics of more massive primeval dwarf galaxies
that merged and disrupted to form the proto-Galaxy.
