Imprints of neutrino-pair flavor conversions on nucleosynthesis in ejecta from neutron-star merger remnants

Abstract: The remnant of neutron star mergers is dense in neutrinos. By employing inputs from one hydrodynamical simulation of a binary neutron star merger remnant with a black hole of $3\ M_\odot$ in the center, dimensionless spin parameter $0.8$ and an accretion torus of $0.3\ M_\odot$, the neutrino emission properties are investigated as the merger remnant evolves. Initially, the local number density of $\bar{\nu}_e$ is larger than that of $\nu_e$ everywhere above the remnant. Then, as the torus approaches self-regulated equilibrium, the local abundance of neutrinos overcomes that of antineutrinos in a funnel around the polar region. The region where the fast pairwise flavor conversions can occur shrinks accordingly as time evolves. Still, we find that fast flavor conversions do affect most of the neutrino-driven ejecta. Assuming that fast flavor conversions lead to flavor equilibration, a significant enhancement of nuclei with mass numbers $A>130$ is found as well as a change of the lanthanide mass fraction by more than a factor of a thousand. Our findings hint towards a potentially relevant role of neutrino flavor oscillations for the prediction of the kilonova (macronova) lightcurves and motivate further work in this direction.