Chemical evolution and kilonova implications of post-merger accretion disk winds

Several gamma-ray bursts (GRBs) have recently been associated with kilonova emission. We study the mechanisms that account for this effect by means of the radioactive decay of elements synthesized in accretion disk winds. We modeled r-process nucleosynthesis in the accretion disk wind system associated with the prompt GRB phase. We computed the time-dependent general relativistic magnetohydrodynamical (GRMHD) evolution of a GRB central engine in which the newly formed black hole accretes mass from the post-merger remnant. We explored the wind properties over a range of initial system parameters and study representative cases of compact binary merger progenitors. We computed a suite of 2D and 3D GRMHD numerical simulations with a tabulated three-parameter equation of state that allows for the evolution of chemical composition in the accretion flow. We accounted for neutrino emission by incorporating a leakage scheme in which the neutrino optical depth was calculated along the radial rays. We parametrized the optically thick and thin tori with different values of the pressure maximum and disk entropy, while we parametrized the strength of large-scale poloidal magnetic fields according to the chosen gas-to-magnetic pressure ratio. To probe the winds, we followed particle trajectories. From these simulations, we derived the nucleosynthetic yields of heavy elements in the outflows, and we mapped the regions of lanthanide-rich and lanthanide-poor ejecta. We find that the outflow carries a high-mass of neutron-rich material expanding at mildly relativistic velocities. Our accretion disks operating in the standard and normal accretion (SANE) mode can power GRB jets via neutrino annihilation if the disk-to-black hole mass ratio exceeds approximately 0.01 and the black hole spins rapidly. Slowly spinning black holes surrounded by massive post-merger disks can also power these jets and can serve as sites of efficient lanthanide nucleosynthesis, which is responsible for the red or purple kilonova components. Long-duration GRBs are potentially produced by a specific type of merger event with an eccentric orbit, but in this case, their lanthanide fraction is reduced, and the GRB luminosity due to neutrinos is also weak. We find that none of the weakly magnetized post-merger models considered here can reproduce the bright kilonovae associated with long GRBs such as 211211A