The r-process nucleosynthesis in the outflows from short GRB accretion disks

Abstract: Short gamma-ray bursts require a rotating black hole, surrounded by a magnetized relativistic accretion disk, such as the one formed by coalescing binary neutron stars or neutron star - black hole systems. The accretion onto a Kerr black hole is the mechanism of launching a baryon-free relativistic jet. An additional uncollimated outflow, consisting of sub-relativistic neutron-rich material which becomes unbound by thermal, magnetic and viscous forces, is responsible for blue and red kilonova. We explore the formation, composition and geometry of the secondary outflow by means of simulating accretion disks with relativistic magneto-hydrodynamics and employing realistic nuclear equation of state. We calculate the nucleosynthetic r-process yields by sampling the outflow with a dense set of tracer particles. Nuclear heating from the residual r-process radioactivities in the freshly synthesized nuclei is expected to power a red kilonova, contributing independently from the dynamical ejecta component, launched at the time of merger, and neutron-poor broad polar outflow, launched from the surface of the hypermassive neutron star by neutrino wind. Our simulations show that both magnetisation of the disk and high black hole spin are able to launch fast wind outflows ($v/c\sim 0.11-0.23$) with a broad range of electron fraction $Y_{\rm e}\sim 0.1-0.4$, and help explain the multiple components observed in the kilonova lightcurves. The total mass loss from the post-merger disk via unbound outflows is between 2\% and 17\% of the initial disk mass.