Simulating neutron star merger remnant outflows and their electromagnetic signatures

As the world still celebrates the detection of the origin of heavy element nucleosynthesis in the Universe at the debris of a binary neutron star merger, we aim to investigate an equally promising system that has yet to be robustly examined as a potential r-process nucleosynthesis site: Collapsars. Derived by “Collapsing Stars”, Collapsars are the dramatic conclusion of extremely heavy and highly rotating stars that reach the end of their lives and nuclear fusion can no longer support their enormous gravity. The condition of the gas near the black hole in collapsars is similar to that in neutron star mergers, so we expect them to produce heavy elements. Although collapsars have 100 times more mass than the binary neutron star mergers, no one really knows if that heavy element enriched outflows can survive the black hole or make it out of the collapsing star. The accretion disk formed by the collapsing gas consists of neutrons and highly magnetized plasma. This plasma has is embedded with a magnetic field that exists in the pre-collapse star. Magnetic turbulence will cause the gas in the disk to be accreted, bringing that magnetic flux on the blackhole, and creating highly magnetized relativistic jets and outflows. The main goal of this proposal is to simulate the self-consistent formation of such a disk and extend the simulation to longer timescales when the jet can reach the surface of the star and breaks out and the outflows containing the heavy radioactive nuclei might get unbound and enrich the Universe with new elements. Although such a project is highly motivated and of highest importance, the technical difficulties have been insurmountable to overcome. I am in the position to run the highest-resolution simulation of a collapsar system, from the self-consistent formation of the accretion disk, the detailed composition history of the outflows carrying the heavy radioactive nuclei, to the jet reaching the stellar surface and, potentially, punching through it and propagating at the surrounding medium. More specifically: 1) We will initialize the simulation with a dipole magnetic field in the pre-collapse star, as expected in the literature. We will follow the magnetic turbulence and shear twisting of that field into the creation of a predominantly toroidal field, of a relatively weak but more realistic strength. By using extremely high-resolution we will be capable, for the first time, to fully resolve the MRI instability in the disk, for an extended period of time. 2) We will include neutrino (and radiation) transport. By including neutrino transport, we will be in a position to calculate the conversion of neutrons into protons, which changes the composition of the disk and suppress the r-process nucleosythesis to fully investigate how many neutrons survive and heavy elements synthesized in the outflows. 3) Use the Helmholtz equation of state to correctly account for electron/positron degeneracy pressure and Compton interactions at the extreme densities and temperatures at the core of the star. 4) Correctly account for the star’s self-gravity during the gas’ collapse and after, by calculating high-order moments for the gravitational potential of the gas and updating the Kerr metric of the black hole, but also, the black hole’s mass and spin as it keeps feeding on the gas. 5) Using an afterglow code that I have developed, use the data from the simulation in the case of the jet break out to predict the afterglow and compare it to existing NASA telescopes’ data.