The MARTINI platform II. Tracing r-process nucleosynthesis in neutron star mergers with long-lived remnants

Abstract

Binary neutron star (BNS) mergers are among the primary sites of the rapid neutron-capture (r-)process. In mergers with long-lived remnants, sustained neutrino irradiation raises the electron fraction of the disc wind ejecta, modifying the resulting nucleosynthesis pattern. Yields from numerical-relativity simulations with accurate neutrino transport are needed to quantify the contribution of BNS mergers to the Galactic production of heavy elements and to interpret kilonova spectra. We present and publicly release tracer trajectories and nucleosynthesis yields for eight BNS mergers forming long-lived neutron star remnants, obtained from general-relativistic simulations with M1 neutrino transport, spanning two equations of state (DD2 and SFHo), and a range of total masses and mass ratios. Passive tracer particles following the dynamical and wind ejecta were post-processed with the nuclear reaction network WinNet up to 1 Gyr. The yields and tracer trajectories are available on the MARTINI platform. At 1 day, the production of light and first-peak elements (He, Ni, Sr, and Y) is similar across models and is primarily enhanced by the neutrino-irradiated disc wind. The yields of heavy nuclei vary by more than one order of magnitude, with unequal-mass binaries producing the largest amounts owing to their more neutron-rich ejecta. Our He yields, particularly in the wind, exceed those of recent studies based on an M0 neutrino scheme, consistent with the stronger neutrino irradiation resulting from an M1 scheme. At 1 Gyr, all models reproduce the second and third solar r-process peaks, but differ in the first peak, the lanthanide region, the mass range between the first and second peaks, and the trans-lead region, reflecting the electron fraction distribution of the ejecta and the dynamical and wind contributions. Two equal-mass models provide a close match to the solar pattern.