Session X01: Binary Neutron Star Evolution and Post-Merger Physics

Heavy element nucleosynthesis in neutron star mergers

The merger of neutron star binaries is accompanied by the ejection of neutron-rich matter that undergoes r-process nucleosynthesis, enriching our Universe with heavy elements. Since the detection of the kilonova associated with the neutron star merger event GW170817, we have the first strong evidence that such mergers are indeed a site where heavy element nucleosynthesis takes place. However, significant uncertainties remain when it comes to the details of heavy element production, with respect to both nuclear physics processes and hydrodynamical modeling. The post-merger evolution of the merger remnant is the most uncertain piece of the problem, and research on this phase is undergoing intense development due to its importance for linking numerical simulations and astrophysical observations. In this talk, I will review our current understanding of r-process nucleosynthesis in neutron star mergers. I will discuss outflows from post-merger remnants, various mass ejection mechanisms, and the role of neutrinos in setting nucleosynthesis yields. Detailed and accurate modeling of the post-merger phase is vital for making optimal use of multi-messenger observations and developing a complete picture of the origin of the heavy elements.   

Neutron-star mergers and r-process nucleosynthesis

Gravitational-wave detectors have transformed the way we observe the Universe. Together with ground and space-based electromagnetic observatories, they have provided key insights into the long-standing question of how the heavy elements in the periodic table are synthesized. I will discuss recent results from simulations of neutron-star mergers with an emphasis on post-merger physics and electromagnetic observables. In particular, I will discuss recent results at the interface of numerical relativity and nuclear astrophysics, formulate a conjecture regarding the main r-process production site, highlight some recent ideas on applications of these concepts in other astrophysical systems, and point out how multi-messenger astronomy can lead to the answer of a >70-year old fundamental question: How does the Universe create the heaviest elements?   

Studying the Equation of State in the Post-Merger Phase of a Binary Neutron star Coalescence

The post-merger phase of a binary neutron star coalescence is a promising new laboratory for studying the dense-matter equation of state (EoS), which remains uncertain despite significant recent progress from multi-messenger observations of the inspiral of GW170817, X-ray constraints on neutron star radii, and laboratory and theoretical efforts. In this talk, I will provide an overview of current EoS constraints, and I will discuss some of the ways in which uncertainties in the EoS can influence the post-merger evolution of neutron star binaries. In particular, I will present a new set of neutron star merger simulations, which explore these uncertainties using a parametrized framework to calculate the microphysical EoS. I will discuss the imprint of systematically varying the nuclear symmetry energy on the post-merger properties. I will also present new quasi-universal relations between the post-merger gravitational waves and the properties of the neutron star EoS, which can be used, in tandem with inspiral observations, to simultaneously constrain both the characteristic radius and the slope of the neutron star mass-radius relation.