Although the rapid neutron-capture process , or r-process , is fundamentally important for explaining the origin of approximately half of the stable nuclei with A > 60 , the astrophysical site of this process has not been identified yet .
Here we study r-process nucleosynthesis in material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars ( NSs ) and within milliseconds afterwards .
For the first time we make use of relativistic hydrodynamical simulations of such events , defining consistently the conditions that determine the nucleosynthesis , i.e. , neutron enrichment , entropy , early density evolution and thus expansion timescale , and ejecta mass .
We find that 10 ^ { -3 } – 10 ^ { -2 } M _ { \odot } are ejected , which is enough for mergers to be the main source of heavy ( A \gtrsim 140 ) galactic r-nuclei for merger rates of some 10 ^ { -5 } yr ^ { -1 } .
While asymmetric mergers eject 2–3 times more mass than symmetric ones , the exact amount depends weakly on whether the NSs have radii of \sim 15 km for a “ stiff ” nuclear equation of state ( EOS ) or \sim 12 km for a “ soft ” EOS .
R-process nucleosynthesis during the decompression becomes largely insensitive to the detailed conditions because of efficient fission recycling , producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140 .
Estimating the light curve powered by the radioactive decay heating of r-process nuclei with an approximative model , we expect high emission in the B-V-R bands for 1–2 days with potentially observable longer duration in the case of asymmetric mergers because of the larger ejecta mass .