Initialization of Binary Neutron Star Orbits Using External Potential Relaxation Scheme

Simulating neutron-star binary mergers is important for probing our knowledge of fundamental physics. Investigation into the equation of state of cold, dense nuclear matter, gaining insight into r-process nucleosynthesis during kilonovae, and gravitational-wave signal interpretation are among the many research pursuits that benefit from the numerical study of compact-star binaries with computational fluid dynamics codes. Merger simulations require accurate initial conditions in regard to the shapes of each star in orbit to correctly model the inspiral phase. In this work we demonstrate two different methods for preparing initial conditions of binary neutron-star systems for Newtonian smoothed particle hydrodynamics simulations. The first method simply assigns particle velocities based on spin, angular momentum, and separation of the stars. The binary is then evolved without the inclusion of gravitational-wave emission, i.e. at a fixed orbital distance, until the stars have relaxed to the physically correct shapes. The second method relaxes the stars within an external potential which emulates the forces experienced by the stars in a frame that is corotating with the binary. The forces deform each star to the configuration they should have given a particular spin and separation to the binary partner. We explore and compare the two different schemes e.g. in terms of their accuracy and computational efficiency. Future work will involve the application of the methods to compact-star binaries with solid components in the crust or core and their impact on the inspiral phase.