We investigate \beta -interactions of free nucleons and their impact on the electron fraction ( Y _ { e } ) and r-process nucleosynthesis in ejecta characteristic of binary neutron star mergers ( BNSMs ) .
For that we employ trajectories from a relativistic BNSM model to represent the density-temperature evolutions in our parametric study .
In the high-density environment , positron captures decrease the neutron richness at the high temperatures predicted by the hydrodynamic simulation .
Circumventing the complexities of modelling three-dimensional neutrino transport , ( anti ) neutrino captures are parameterized in terms of prescribed neutrino luminosities and mean energies , guided by published results and assumed as constant in time .
Depending sensitively on the adopted \nu _ { e } - \bar { \nu } _ { e } luminosity ratio , neutrino processes increase Y _ { e } to values between 0.25 and 0.40 , still allowing for a successful r-process compatible with the observed solar abundance distribution and a significant fraction of the ejecta consisting of r-process nuclei .
If the \nu _ { e } luminosities and mean energies are relatively large compared to the \bar { \nu } _ { e } properties , the mean Y _ { e } might reach values > 0.40 so that neutrino captures seriously compromise the success of the r-process .
In this case , the r-abundances remain compatible with the solar distribution , but the total amount of ejected r-material is reduced to a few percent , because the production of iron-peak elements is favored .
Proper neutrino physics , in particular also neutrino absorption , have to be included in BNSM simulations before final conclusions can be drawn concerning r-processing in this environment and concerning observational consequences like kilonovae , whose peak brightness and color temperature are sensitive to the composition-dependent opacity of the ejecta .