Chemical Evolution and Kilonova Implications of Post-Merger Accretion Disk Winds

Abstract

Several gamma ray bursts have recently been associated with a kilonova emission. We study the mechanisms which could account for this effect, by means of radioactive decay of elements synthesized in accretion disk wind. We model the r-process nucleosynthesis in the accretion disk wind system, asscociated with the prompt GRB phase. We compute the time-dependent GR MHD evolution of a GRB central engine where the newly formed black hole is accreting the mass from post-merger remnant. We explore the wind properties, for a range of the initial parameters of the system, and study representative cases for compact binary merger progenitors. We compute a suite of 2D and 3D time-dependent General Relativistic numerical simulations with a tabulated 3-parameter equation of state that allows for evolution of chemical composition evolution of the accretion flow. The neutrino emission is accounted for by incorporating the leakage scheme, where neutrino optical depth is calculated along the radial rays. We parameterize the optically thick and thin tori with different values of the pressure maximum and entropy in the disk, while the strength of large-scale poloidal magnetic fields is parameterized according to the chosen gas-to-magnetic pressure ratio. To probe the winds, we follow the particle trajectories. Upon this, we derive the nucleosynthetic yields of heavy elements in the outflows, and we map the regions of Lanthanide rich and poor ejecta. We find that the outflow carries high mass of neutron rich material expanding with mildly relativistic velocities. Our accretion disks operating under the SANE mode can power the GRB jets via neutrino annihilation, if the disk to BH mass ratio is larger than about 0.01 and the black hole is spinning. Slowly spinning black holes surrounded by massive post-merger disks can power these jets, and also be the sites of efficient nucleosynthesis of Lanthanides.