Correlations of r-Process Elements in Very Metal-Poor Stars as Clues to
  their Nucleosynthesis Sites
  Various nucleosynthesis studies have pointed out that the r-process elements
in very metal-poor (VMP) halo stars might have different origins. By means of
familiar concepts from statistics (correlations, cluster analysis, rank tests
of elemental abundances), we look for causally correlated elemental abundance
patterns and attempt to link them to astrophysical events. Some of these events
produce the r-process elements jointly with iron, while others do not have any
significant iron contribution. In the early stage of our Galaxy, at least three
r-process nucleosynthesis sites have been active. The first two produce and
eject iron and the majority of the lighter r-process elements. We assign them
to two different types of core-collapse events, not identical to regular
core-collapse supernovae (CCSNe), which produce only light trans-Fe elements.
The third category is characterized by a strong r-process and responsible for
the major fraction of the heavy main r-process elements without a significant
co-production of Fe. It does not appear to be connected to CCSNe, in fact the
Fe found in the related r-process enriched stars must come from previously
occurring CCSNe. The existence of actinide boost stars indicates a further
division among strong r-process sites. We assign these two strong r-process
sites to neutron star mergers without fast black hole formation and to events
where the ejecta are dominated by black hole accretion disk outflows.
Indications from the lowest-metallicity stars hint at a connection to massive
single stars (collapsars) forming black holes in the early Galaxy.
