Formulating bulk viscosity for neutron star simulations

TL;DR

This paper discusses how to incorporate bulk viscosity into neutron star merger simulations to better understand the physics from gravitational and electromagnetic signals, emphasizing the importance of nuclear reactions.

Contribution

It introduces a framework for modeling bulk viscosity in neutron star simulations considering nuclear reactions and different physical regimes.

Findings

Bulk viscosity is essential for accurate neutron star merger modeling.

Different regimes require distinct assumptions for nuclear reaction effects.

Connecting bulk viscosity with turbulence models enhances simulation fidelity.

Abstract

In order to extract the precise physical information encoded in the gravitational and electromagnetic signals from powerful neutron-star merger events, we need to include as much of the relevant physics as possible in our numerical simulations. This presents a severe challenge, given that many of the involved parameters are poorly constrained. In this paper we focus on the role of nuclear reactions. Combining a theoretical discussion with an analysis connecting to state-of-the-art simulations, we outline multiple arguments that lead to a reactive system being described in terms of a bulk viscosity. The results demonstrate that in order to properly account for nuclear reactions, future simulations must be able to handle different regimes where rather different assumptions/approximations are appropriate. We also touch upon the link to models based on the large-eddy-strategy required to capture turbulence.