Halted-Pendulum Relaxation: Application to White Dwarf Binary Initial Data

Studying compact star binaries and their mergers is integral to modern astrophysics. In particular, binary white dwarfs are associated with Type Ia supernovae, used as standard candles to measure the expansion of the Universe. Today, compact-star mergers are typically studied via state-of-the-art computational fluid dynamics codes. One such numerical techniques, Smoothed Particle Hydrodynamics (SPH), is frequently chosen for its excellent mass, energy, and momentum conservation. Furthermore, the natural treatment of vacuum and the ability to represent highly irregular morphologies make SPH an excellent tool for the numerical study of compact-star binaries and mergers. However, for many scenarios, including binary systems, the outcome simulations are only as accurate as the initial conditions. For SPH, it is essential to ensure that particles are distributed semi-regularly, correctly representing the initial density profile. Additionally, particle noise in the form of high-frequency local motion and low-frequency global dynamics must be damped out. Damping the latter can be as computationally intensive as the actual simulation. Here, we discuss a new and straightforward relaxation method, Halted-Pendulum Relaxation (HPR), to remove the global oscillation modes of SPH particle configurations. In combination with effective external potentials representing gravitational and orbital forces, we show that HPR has an excellent performance in efficiently relaxing SPH particles to the desired density distribution and removing global oscillation modes. We compare the method to frequently used relaxation approaches such as gravitational glass, increased artificial viscosity, and Weighted Voronoi Tesselations, and test it on a white dwarf binary model at its Roche lobe overflow limit

Abundances and Transients from Neutron Star–White Dwarf Mergers

We systematically investigate the mergers of neutron star–white dwarf binaries from beginning to end, with a focus on the properties of the inflows and outflows in accretion disks and their electromagnetic emissions. Using population synthesis models, we determine a subset of these binaries in which the white dwarf companion undergoes unstable mass transfer and complete tidal disruption, forming a large accretion disk around the neutron star. The material evolves according to a one-dimensional advection-dominated accretion-disk model with nuclear burning, neutrino emissions, and disk-surface wind ejection. The extreme dynamics of the entire process have proven difficult to analyze, and thus currently, the properties are poorly understood. The outflows from the mergers are iron- and nickel-rich, giving rise to optical and infrared emissions powered by the decay of the radioactive iron-type isotopes, calculated via the SuperNu light-curve code. We find these systems capable of powering bright, yet short-lived, optical transients with the potential to power gamma-ray bursts