Neutron star ( NS ) merger events can eject considerable amounts of neutron-rich matter , whose expansion allows heavy nuclei up to uranium and thorium to be formed by the rapid neutron-capture process .
The radioactive decay of the products of this r-process heats the ejecta material and allows it to become potentially observable as a source of thermal electromagnetic radiation .
Here we investigate for the first time systematically the dynamical mass ejection during and shortly after the NS collision in dependence on the uncertain properties of the nuclear equation of state ( EoS ) by employing a set of 40 representative , microphysical high-density EoSs in relativistic , hydrodynamical merger simulations .
The NS compactness , which is a characteristic EoS property and can be expressed by the radius R _ { 1.35 } of nonrotating NSs of 1.35 M _ { \odot } , turns out to be a crucial parameter that determines the ejecta mass .
NSs with smaller radii R _ { 1.35 } ( “ soft ” EoS ) collide more violently and eject systematically higher masses .
These range from \sim 10 ^ { -3 } M _ { \odot } to \sim 10 ^ { -2 } M _ { \odot } for symmetric 1.35-1.35 M _ { \odot } binaries with R _ { 1.35 } between 16 km and 11 km , and from \sim 5 \times 10 ^ { -3 } M _ { \odot } to \sim 2 \times 10 ^ { -2 } M _ { \odot } for asymmetric 1.2-1.5 M _ { \odot } binaries .
Correspondingly , the bolometric peak luminosities of the optical transients of symmetric ( asymmetric ) mergers vary between 3 \times 10 ^ { 41 } erg s ^ { -1 } and 14 \times 10 ^ { 41 } erg s ^ { -1 } ( 9 \times 10 ^ { 41 } erg s ^ { -1 } and 14.5 \times 10 ^ { 41 } erg s ^ { -1 } ) and are reached with effective spectral temperatures of 13.000–19.000 K on typical timescales between \sim 2 h and \sim 12 h. If these electromagnetic signals with absolute bolometric magnitudes from - 15.0 to - 16.7 can be measured , the tight correlation of their properties with those of the merging NSs might provide valuable constraints on the high-density EoS .
The wide range of ejecta velocities and kinetic energies in our models ( between 15 % and 50 % of the speed of light and between \sim 5 \times 10 ^ { 49 } erg and 10 ^ { 51 } erg , respectively ) suggests variations by factors of several 100 for the peak of the radio flux expected from the ejecta interaction with the circumstellar medium .
In contrast , despite differences in the ejecta dynamics and thermodynamics , the r-process nucleosynthesis exhibits a remarkable robustness .
In all investigated cases more than 93 % of the ejecta mass are converted to neutron-rich nuclei with nuclear mass numbers A \gtrsim 130 and a uniform , nearly solar abundance pattern .
By the r-process content of the Galaxy and the average production per event the Galactic merger rate is limited to 4 \times 10 ^ { -5 } yr ^ { -1 } ( 4 \times 10 ^ { -4 } yr ^ { -1 } ) for a soft ( stiff ) NS EoS , if NS mergers are the main source of heavy r-nuclei .
The production ratio of radioactive ^ { 232 } Th to ^ { 238 } U , which we determine for the first time in the discussed context , attains a stable value of 1.64–1.67 , which does not exclude NS mergers as potential sources of heavy r-material in the most metal-poor stars .