Dynamics of Dense Matter in Neutron Star Mergers

We investigate proton fraction changing processes in neutron star mergers such as neutron decay, electron capture, and their inverse reactions. Our calculations use an energy-dependent neutrino population from a neutron star merger simulation with a Monte Carlo neutrino scheme. This work goes beyond typical methods that assume neutrinos either escape the system completely or form a Fermi-Dirac distribution with some lepton fraction.

Weak interactions play an essential role in neutron star mergers by changing the proton fraction and thereby emitting and absorbing neutrinos and antineutrinos. Understanding how quickly proton fraction equilibration happens is essential for groups doing simulations of neutron star mergers to know which processes are relevant to include. If the timescale of proton fraction equilibration occurs on a similar timescale as density oscillations in a merger there will be viscous effects. This can affect the post-merger gravitational waveform, which we will probe using next-generation gravitational-wave observatories.