UC Santa Cruz

This thesis focuses on describing what happens when a binary neutron star merges. It uses numerical simulations to study the different dominant physics before and after the merger. In August 17, 2017, the first binary neutron star merger was observed by LIGO/Virgo, the gravitational wave observatory. Great efforts by astronomers all around the globe allowed for the electromagnetic follow-up of the event. The discovery, named GW170817, showed evidence that binary neutron stars can produce gamma-ray bursts, and also synthesize heavy elements, such as gold and platinum.

This thesis studies the intriguing question of how binary neutron stars have been able to assemble and merge within the timespan the Universe has existed. One of the preferred channels is called common envelope evolution. This happens when, in a binary of massive stars, the more massive star (the primary) evolves, expands and engulfs its companion. Drag forces from the envelope slow down the companion and tighten the orbit. The energy from the orbit will be transferred to the envelope and will potentially have enough energy to unbind the envelope. The result is the very rapid creation of a tighter binary. This thesis studies the accretion flows around the companion, given that the envelope has a gradient. Material will be focused on the companion (the secondary), but due to the density gradient, there will be an angular momentum barrier that inhibits accretion onto the secondary. We study how different microphysical parameters of the envelope affect the angular momentum redistribution.

It is widely believed that the merger of GW170817 was a hyper-massive neutron star that after a certain delay time collapsed into a black hole surrounded by an accretion disk. Material in the disk will lose angular momentum due to magnetic stresses, eventually falling into the black hole and driving a relativistic, beamed jet. The jet will interact with winds launched during the hyper-massive neutron star phase. This thesis studies jet-wind interactions, since they lead to non-thermal emission. We used numerical simulations in order to constrain certain parameters of the binary itself, such as the delay time between the merger and the collapse to a black hole.

Several other outfows are expected to occur after the merger, including an outflow driven from magnetic stresses in the accretion disk. The outflow will cool, expand and create heavy elements via the r-process. This elements will then radioactivelydecay and be observed. In the inner regions of the disk, the high temperatures and densities will ignite the creation of neutrinos and anti-neutrinos via weak reactions. In the outer region of the disk, free nucleons will recombine into alpha-particles, which will release nuclear binding energy. Both the neutrinos and the recombination energy will greatly affect the outflow, especially the electron fraction. The electron fraction is key in determining the final abundance of elements. This thesis studies the impact that neutrinos and a finite-temperature equation of state have on the outflow.