r -process Heating Feedback on Disk Outflows from Neutron Star Mergers

Abstract Neutron star mergers produce r -process elements, with yields that are sensitive to the kinematic and thermodynamic properties of the ejecta. These ejecta properties are potentially affected by dynamically important feedback from r -process heating, which is usually not coupled to the hydrodynamics in postmerger simulations modeling the ejecta launching and expansion. The multimessenger detection of GW170817 showed the importance of producing reliable ejecta predictions to maximize the diagnostic potential of future events. In this paper, we develop a prescription for including r -process heating as a source term in the hydrodynamic equations. This prescription depends on local fluid properties and the Y e history as recorded by dedicated tracer particles, which exchange information with the grid using the cloud-in-cell method. The method is implemented in long-term viscous hydrodynamic simulations of accretion disk outflows to investigate its feedback on ejecta properties. We find that r -process heating can increase the unbound disk ejecta mass by ∼10% relative to a baseline case that only considers alpha particle recombination. Nuclear heating also enhances the radial velocity of the ejecta with Y e < 0.25 by up to a factor of 2 while concurrently suppressing marginally bound convective ejecta.