Nucleosynthesis on the aftermath of neutron star mergers: The creation of the first r-process peak

The rapid neutron capture nucleosynthesis (r-process) is responsible for producing about half of the solar abundances of heavy elements. The site of the r-process was unknown until recent observations. The identification of the kilonova in the aftermath of GW170817 established binary neutron star mergers (BNS) as a site for the r-process. The early blue color of the emission indicates the production of light, lanthanide-free, elements and can be linked to a "weak" r-process nucleosynthesis scenario. We explored nucleosynthesis under a broad set of entropy and electron fraction conditions using GSINet. We concluded that with conditions realized in BNS, an abundance pattern resembling the first r-process peak could be created, accompanied by a peak at A ≈ 50. The origin of the peak at A ≈ 50 was found to be material that could not escape the Z = 20 closed protons shell. More specifically, we found that in the absence of α-particles, the isotopes of Ca with A = 54 − 58 were the critical isotopes regulating the flow towards higher A for low entropy (S ≈ 15 kb/baryon) and moderate electron fraction (0.35 ≤ Ye ≤ 0.40) conditions. We investigated the impact of nuclear properties (namely masses, β-decays, β-delayed neutron emissions) on the production of the first r-process peak under these conditions. To quantify the impact of nuclear masses on the final abundance pattern, we studied the impact of nuclear mass on calculations of (n, γ) reaction rates. We calculated sets of reaction rates corresponding to the mass uncertainty bands. We assumed the true mass to lie between the central AME16 value and its uncertainty. For unknown masses, the FRDM mass model was used with a 1 MeV uncertainty. Using the TALYS nuclear reaction code, we performed large-scale calculations of reaction rates corresponding to the region responsible for creating the first r-process peak. The calculated reaction rates were then used in GSINet to calculate abundances. The impact of each modified mass on the abundance pattern was evaluated. We found that 55,56,57,58Sc,77Ni, and 83Zn are the nuclei with the most significant impact on the abundance pattern. Furthermore, the impact of mass uncertainties on radioactive heating was evaluated. We identified and presented the nuclei responsible for the heating production. We concluded that current uncertainties in mass values could lead to uncertainties in heating up to a factor of two. We observed that the shape of the abundance pattern does not drastically change under changes in mass values. Masses were found to impact the peak local structure. A specific study of the impact of the masses of 84,85Ga to the local maximum at A = 84 was presented. Finally, we evaluated the impact of β-decays, finding no significant variations on the final abundance pattern