LIGO Document P1500204-v1

Parametrization of the effect of weak interactions on the production of heavy elements in binary neutron star mergers

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Recent research has shown that material ejected during binary neutron star mergers (BNSMs)
is a likely source of about half of our galaxy’s heavy nuclei created. These nuclei are synthe-
sized via rapid neutron capture, or the r-process. We investigate the effect of positron, electron,
electron neutrino and antineutrino capture reactions on the r-process in BNSM ejecta, which is
uncertain and has only received significant attention recently. To do this, we modify the nuclear
reaction network SkyNet to include the aforementioned interactions. Then, we parametrize the
neutrino luminosities and entropy of the outflow to study their effect on the evolution of electron
abundance, isotopic abundances, and reaction rates in the BNSM ejecta. We then compare our
results to the observed solar abundances of r-process elements to gauge the accuracy of our sim-
ulations. Results show that, for a reasonable set of parameters, neutrino luminosities at or above
1053 ergs/s may significantly limit the production of heavy elements of mass number >120. We
also find that increasing entropy decreases the sensitivity of the electron abundance to neutrino
luminosities. Similarly, at high neutrino luminosities (around 1053 ergs/s), the final electron abun-
dance becomes less sensitive to entropy. The increase in electron abundance caused by increasing
luminosity and entropy is also related to how rapidly the ejecta expands away from the collision,
parametrized as t. Some further research may investigate the ratio of neutrino to antineutrino
luminosity as a parameter, the inability to form heavy elements >200 in our neutrino-including
r-process simulations, and more accurate modeling of neutrino outflow.
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held on 20 Aug 2015 in SCR

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