2017ApJ...835L...3T

Meteoritic abundances of r-process elements are analyzed to deduce the history of chemical enrichment by the r-process, from the beginning of disk formation to the present time in the solar vicinity. Our analysis combines the abundance information from short-lived radioactive nuclei such as ²⁴⁴Pu with the abundance information from stable r-process nuclei such as Eu. These two types of nuclei can be associated with one r-process event and an accumulation of events until the formation of the solar system, respectively. With the help of the observed local star formation (SF) history, we deduce the chemical evolution of ²⁴⁴Pu and obtain three main results: (i) the last r-process event occurred 130-140 Myr before the formation of the solar system; (ii) the present-day low ²⁴⁴Pu abundance as measured in deep-sea reservoirs results from the low recent SF rate compared to ∼4.5-5 Gyr ago; and (iii) there were ∼15 r-process events in the solar vicinity from the formation of the Galaxy to the time of solar system's formation and ∼30 r-process events to the present time. Then, adopting the hypothesis that a neutron star (NS) 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 ∼3.5×10⁶M_☉, which is about 100 times larger than that for supernova ejecta. In addition, the event frequency of r-process production is estimated to be 1 per ∼1400 core-collapse supernovae, which is identical to the frequency of NS mergers estimated from the analysis of stellar abundances.