Two Sites of r-Process Production Assessed on the Basis of the
  Age-tagged Abundances of Solar Twins
  Solar twins, i.e., stars that are nearly identical to the Sun, including
their metallicities, in the solar vicinity show ages widely distributed from
0-10 Gyr. This fact matches the orbital history of solar twins in the new
paradigm of galactic dynamics, in which stars radially move on the disk when
they encounter transient spiral arms. This finding suggests that older twins
were born closer to the Galactic center and traveled a longer distance to reach
their present location, according to the hypothesis that chemical enrichment
occurs more quickly and that solar metallicity is attained on a shorter
timescale with a decreasing Galactocentric distance (R_GC). We show that
abundance patterns covering a wide range of heavy elements for solar twins
sharing similar ages are identical and that their variation among different age
groups can be understood on the basis of the age-R_GC connection within the
framework of Galactic chemical evolution. This study identifies the Galactic
bulge as the birthplace of the oldest solar twins. Based on this scheme, we
find that the relation between [r-process/Fe] and R_GC for the inner Galactic
region is incompatible with the hypothesis of a sole site for r-process
production, that is, neutron star mergers, whose delay time distribution could
be approximated by the power-law form (proportional to t^n). Alternatively,
this relation suggests the presence of two distinct sites for r-process
production: short-lived massive stars, ending with specific core-collapse
supernovae, and neutron star mergers that are heavily inclined to emerge with
longer delay times, as represented by n=0-0.5.
