Neutron Star Mergers and Nucleosynthesis of Heavy Elements

Abstract

Neutron star mergers have been predicted since the 1970's, supported by the discovery of the binary pulsar and the observation of its orbital energy loss, consistent with General Relativity. They are considered as nucleosynthesis sites of the rapid neutron-capture process (r-process), being responsible for making about half of all heavy elements beyond Fe and being the only source of elements beyond Pb and Bi. Detailed nucleosynthesis calculations based on the decompression of neutron-star matter are consistent with solar r-process abundances of heavy nuclei. More recently neutron star mergers have also been identified with short duration Gamma-Ray Bursts via their IR afterglow, only explainable by the opacities of heavy (rather than only Fe-group) nuclei. Two other observations support rare events like neutron star mergers as a dominant scenario for the production of the heaviest r-process nuclei: (a)The discrepancy between the latest admixtures of two long-lived radioactivities ( 60 Fe and 244 Pu) found on earth seems to exclude the origin of the latter from core collapse supernovae. (b)The ratio of [Eu/Fe], with Eu being dominated by r-process contributions, shows a strong scatter in low metallicity stars up to [Fe/H]<-2, arguing for a strongly reduced occurrence rate in comparison to core-collapse supernovae. The high neutron densities in ejected matter permit a violent r-process, encountering fission cycling of the heaviest nuclei in regions far from (nuclear) stability. Uncertainties in nuclear properties, like nuclear masses, betadecay half-lives, fission barriers and fission fragment distributions affect the detailed abundance distributions. The modeling of the astrophysical events depends also on the hydrodynamic treatment, i.e. SPH vs. grid calculations, Newtonian vs. GR approaches, the occurrence of a neutrino wind after the merger and before the emergence of a black hole, and finally the properties of black hole accretion disks. We will discuss the effect of both (nuclear and modelling) uncertainties and conclude that binary compact mergers are probably a or the dominant site of the production of r-process nuclei in our Galaxy. A small caveat exists with respect to explaining the behavior of [Eu/Fe] at lowest metallicities and the question whether neutron star mergers can already contribute at such early times in galactic evolution.2 Thielemann et al.

Probing the Production of Actinides under Different r-process Conditions

“…The shaded area highlights a range 1.5 MeV < S n < 1.7 MeV, where the reaction flow can easily bypass nuclei with mass numbers A = 232 − 238 in the FRDM, as shown in the following. In hot r-process conditions, nuclei that are located on the reaction path can be identified based purely on their two-neutron separation energy, as well as the neutron density n n , and the temperature T (see, e.g., Thielemann et al 2017). The S 2n /2 values predicted by the FRDM mass model in the mass region 220 < A < 260 are shown in Figure 7, together with typical r-process paths favouring nuclei around S 2n /2 = 1.9, 1.7, 1.5, 1.3 MeV (red dots).…”

Galactic r-process Abundance Feature Shaped by Radial Migration

“…The feature of r-process abundance among disk stars, which is represented by the [Eu/Fe] vs. [Fe/H] diagram, has recently attracted attention. The discovery of gravitational waves from the neutron star merger (NSM) GW170817 and the subsequent discovery of multiwavelength electromagnetic counterparts, i.e., kilonova, has identified NSMs as a major source of r-process elements (e.g., Smartt et al 2017;Pian et al 2017;Cowperthwaite et al 2017;Thielemann et al 2017). However, it has been reported that the evolutionary path of [Eu/Fe] predicted by NSMs is incompatible with the observed trend of disk stars due to a delayed r-process enrichment (Hotokezaka et al 2018).…”

First Exploration of Neutron Shell Structure below Lead and beyond N=126

“…The robustness of the N = 126 neutron shell closure plays a crucial role in the nucleosynthesis of the actinides [3][4][5][6][7]. The recent observation of a neutron star merger has provided a new focus of interest [8,9], suggesting a possible astrophysical environment for r-process nucleosynthesis [10][11][12][13].…”