Astrophysical transients and compact objects

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This research program aims to understand the physics of cosmic explosions related to stellar-mass compact objects. The initial focus is on neutron star mergers and core-collapse supernovae. These explosions operate under extreme conditions not achievable in laboratories on Earth, and thereby connect to frontier questions in physics such as the properties of matter at supra-nuclear densities, the origin of heavy elements, the nature of gravity, and the properties of neutrinos. The long-term goal of the program is to improve our theoretical understanding of this class of cosmic explosions through scientific computing, thereby contributing to more reliable observational predictions and to test physical theories. Numerical methods are required given the complexity of the processes involved. Highly-qualified personnel trained in high-performance computing and advanced analysis techniques will be one of the many beneficial results of this program. Neutron star mergers are likely to be detected in gravitational waves in the period 2017-2021 by Advanced LIGO/Virgo. Concurrent detection of electromagnetic emission will significantly decrease uncertainties in the gravitational wave signal and provide a wealth of additional information. The first short-term goal of the program is to improve predictions for the most easily detectable electromagnetic signal generated by these events: a supernova-like transient powered by the radioactive decay of r-process elements (a so-called kilonova). To this end, multi-dimensional, multi-physics, time-dependent simulations of the merger remnant will be conducted. These simulations will systematically address current uncertainties in modeling, thus improving observational and nucleosynthetic predictions for the global observational community. The explosion mechanism of core-collapse supernovae remains an unsolved theoretical problem, despite a firm observational association between these supernovae and the death of massive stars. These explosions enrich the Universe with heavy elements, and are the birth sites of neutron stars and stellar-mass black holes. Existing evidence suggests that the explosion mechanism is intrinsically asymmetric. The second short-term goal of the program is to investigate the effect of hydrodynamic instabilities that break spherical symmetry: the Standing Accretion Shock Instability (SASI) and neutrino-driven convection. Multi-dimensional simulations at different levels of sophistication will be carried out to clarify the physics of these instabilities in new regimes and the resulting observational consequences. The results will have implications for the expected neutrino- and gravitational wave signal from the next Galactic core-collapse supernova, and will shed light on related astrophysical issues such as the distribution of pulsar spins and kicks at birth.