General Relativistic Moving-mesh Simulations of Neutron-Star Mergers

The ERC Relativistic Astrophysics Group at the GSI Helmholtz Centre in Darmstadt focuses on hydrodynamical computer simulations of neutron-star mergers (see movie). The main goal of the group is to link obseravbles of neutron-star mergers to fundamental physics questions. This concerns for instance the properties of high-density nuclear matter and the transition to a quark-gluon plasma at finite chemical potential, which may be extracted from gravitational-wave signals of neutron-star mergers. The team also works on the formation of heavy elements through the rapid neutron-capture process in the ejecta of neutron-star mergers. These nucleosynthetic processes power the emission of kilonovae - the electromagnetic counterparts of colliding neutron stars. The team develops novel numerical methods to extract information about the underlying mass ejection and element formation from observations. The work of the group is supported by an ERC Starting Grant (grant agreement No. 759253). The team is lead by Priv.-Doz. Dr. Andreas Bauswein.

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Research Highlight: Black-hole torus evolution

Material ejected into the interstellar medium by a neutron-star merger can be extremely massive and neutron rich and could therefore represent a major site of the rapid neutron-capture (r-) process that is responsible for the creation many heavy elements, such as Gold and Uranium. Whether or not neutron-star mergers are a significant, or even dominant, source of r-process elements depends sensitively on the mass and level of neutron richness of the material released during and after the merger. A new study now investigated the conditions to achieve a high neutron density in outflows from a black-hole disk system, which is often left behind after a neutron-star merger. The study systematically clarifies the roles of individual weak interactions between neutrons, protons, electrons, and neutrinos for creating neutron-rich conditions. It is found that disks reach the highest neutron densities if they are massive enough for electrons to be degenerate, but not so massive for neutrino absorption to become dominant. These conditions happen to be realized exactly for disk masses in the range produced after typical neutron-star mergers. This provides strong support to the idea that neutron-star mergers are major r-process sites. (See also the press release).

Research Highlight: Black-hole formation in Neutron Star Mergers

Neutron stars are the densest stellar objects in the Universe. They cannot be arbitrarily massive because beyond some threshold the gravitational attraction becomes too strong and the gravitational collapse to a black hole is unavoidable. Therefore, the collision of two stable neutron stars can lead to the formation of a black hole. The outcome basically depends on the total mass of the system and whether or not it exceeds the threshold for black-hole formation. In a recent study, which appeared in Physical Review Letters, we investigated the outcome of neutron star mergers and under which conditions they result in a black hole. Interestingly, the exact threshold for the gravitational collapse depends sensitively on the properties of nuclear matter. In turn, these dependencies imply that the outcome of neutron star mergers can inform about still unknown properties of high-density matter. (See also the press release)

Research Highlight: Identifying a First-Order Phase Transition in Neutron-Star Mergers through Gravitational Waves

The transition from ordinary nucleonic matter to deconfined quark matter at higher densities is still mysterious. Possibly neutron stars feature a core of quark matter in their center. This offers the opportunity to learn about the hadron-quark pahse transition from observations of neutron stars. Based on computer simulations we identified an unambiguous signature of the presence of quark matter in the gravitational-wave signal of neutron-star mergers. The appearance of quark matter can be detected by comparing the signal of the premerger stage with the characteristic frequencies inferred from the postmerger phase. A significant increase of the postmerger gravitational-wave frequencies would be indicative of the occurrence of deconfied quark matter. The results of the study have been published in Physical Review Letters (see also the press release).