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We investigate beta-interactions of free nucleons and their impact on theelectron fraction (Y_e) and r-process nucleosynthesis in ejecta characteristicof binary neutron star mergers (BNSMs). For that we employ trajectories from arelativistic BNSM model to represent the density-temperature evolutions in ourparametric study. In the high-density environment, positron captures decreasethe neutron richness at the high temperatures predicted by the hydrodynamicsimulation. Circumventing the complexities of modelling three-dimensionalneutrino transport, (anti)neutrino captures are parameterized in terms ofprescribed neutrino luminosities and mean energies, guided by published resultsand assumed as constant in time. Depending sensitively on the adoptedneutrino-antineutrino luminosity ratio, neutrino processes increase Y_e tovalues between 0.25 and 0.40, still allowing for a successful r-processcompatible with the observed solar abundance distribution and a significantfraction of the ejecta consisting of r-process nuclei. If the electron neutrinoluminosities and mean energies are relatively large compared to theantineutrino properties, the mean Y_e might reach values >0.40 so that neutrinocaptures seriously compromise the success of the r-process. In this case, ther-abundances remain compatible with the solar distribution, but the totalamount of ejected r-material is reduced to a few percent, because theproduction of iron-peak elements is favored. Proper neutrino physics, inparticular also neutrino absorption, have to be included in BNSM simulationsbefore final conclusions can be drawn concerning r-processing in thisenvironment and concerning observational consequences like kilonovae, whosepeak brightness and color temperature are sensitive to thecomposition-dependent opacity of the ejecta.

Impact of weak interactions of free nucleons on the r-process in dynamical ejecta from neutron-star mergers