Strangeness-changing Rates and Hyperonic Bulk Viscosity in Neutron Star Mergers

TL;DR

This paper calculates strangeness-changing rates and hyperonic bulk viscosity in neutron star merger matter, emphasizing the importance of meson exchange processes and their temperature dependence for dissipation mechanisms.

Contribution

It introduces a comprehensive numerical evaluation of strangeness-changing rates including meson exchange processes, extending previous models beyond the Fermi surface approximation.

Findings

Hyperon bulk viscosity is negligible at MeV temperatures.

Meson exchange processes dominate strangeness-changing rates.

Bulk viscosity becomes significant at keV temperatures.

Abstract

In this paper we present a computation of the rates of strangeness-changing processes and the resultant bulk viscosity in matter at the densities and temperatures typical of neutron star mergers. To deal with the high temperature in this environment we go beyond the Fermi surface approximation in our rate calculations and numerically evaluate the full phase space integral. We include processes where quarks move between baryons via meson exchange: these have generally been omitted in previous analyses but provide the dominant contribution to the rates of strangeness-changing processes and the bulk viscosity. The calculation of these rates is an essential step towards any calculation of dissipation mechanisms in hyperonic matter in mergers. As one application, we calculate the dissipation times for density oscillations at the frequencies seen in merger simulations. We find that hyperon bulk viscosity for temperatures in the MeV regime can probably be neglected in this context, but becomes highly relevant for keV-range temperatures.