Chemical evolution of 244Pu in the solar vicinity and its implication
  for the properties of r-process production
  Meteoritic abundances of r-process elements are analyzed to deduce the
history of chemical enrichment by r-process from the beginning of disk
formation to the present time in the solar vicinity, by combining the abundance
information from short-lived radioactive nuclei such as 244Pu with that from
stable r-process nuclei such as Eu. These two types of nuclei can be associated
with one r-process event and cumulation of events till formation of the solar
system, respectively. With help of the observed local star formation history,
we deduce the chemical evolution of 244Pu and obtain three main results: (i)
the last r-process event occurred 130-140 Myr before formation of the solar
system, (ii) the present-day low 244Pu abundance as measured in deep sea
reservoirs results from the low recent star formation rate compared to ~4.5 - 5
Gyr ago, and (iii) there were ~15 r-process events in the solar vicinity from
formation of the Galaxy to the time of solar system formation and ~30 r-process
events to the present time. Then, adopting a reasonable hypothesis that a
neutron star merger is the r-process production site, we find that the ejected
r-process elements are extensively spread out and mixed with interstellar
matter with a mass of ~3.5 million solar masses, which is about 100 times
larger than that for supernova ejecta. In addition, the event frequency of
r-process production is estimated to be one per about 1400 core-collapse
supernovae, which is identical to the frequency of neutron star mergers
estimated from the analysis of stellar abundances.
