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

TL;DR

This paper introduces a new method to include $r$-process heating feedback in hydrodynamic simulations of neutron star merger ejecta, revealing its significant impact on ejecta mass and velocity.

Contribution

It develops a prescription for incorporating $r$-process heating into hydrodynamic models, improving predictions of ejecta properties in neutron star mergers.

Findings

$r$-process heating increases unbound ejecta mass by ~10%.

Heating doubles the velocity of low-$Y_e$ ejecta.

Heating suppresses marginally-bound convective ejecta.

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 post-merger simulations modeling the ejecta launching and expansion. The multi-messenger 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 on 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 $\sim 10\%$ 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 two, while concurrently suppressing marginally-bound convective ejecta.