On the astrophysical robustness of the neutron star merger r-process

Abstract

In this study we explore nucleosynthesis in the dynamic ejecta of compact binary mergers. We are particularly interested in the question how sensitive the resulting abundance patterns are to the parameters of the merging system. Therefore, we systematically investigate combinations of neutron star masses in the range from 1.0 to 2.0 M⊙ and, for completeness, we compare the results with those from two simulations of a neutron star black hole merger. The ejecta masses vary by a factor of 5 for the studied systems, but all amounts are (within the uncertainties of the merger rates) compatible with being a major source of the cosmic r‐process. The ejecta undergo robust r‐process nucleosynthesis which produces all the elements from the second to the third peak in close‐to‐solar ratios. Most strikingly, this r‐process is extremely robust, and all 23 investigated binary systems yield practically identical abundance patterns. This is mainly the result of the ejecta being extremely neutron rich (Ye ≈0.04) and the r‐process path meandering along the neutron drip line so that the abundances are determined entirely by nuclear rather than astrophysical properties. While further questions related to galactic chemical evolution need to be explored in future studies, we consider this robustness together with the ease with which both the second and third peak are reproduced as strong indications that compact binary mergers are prime candidates for the sources of the observed unique heavy r‐process component.

Origin of r-Process Elements and Galactic Chemodynamical Evolution

“…Recent studies [29][30][31][32]35] have indicated that the r process in NSMs produces a final abundance pattern that can be similar to the solar r-process abundances, but only for heavier A>130 nuclei.…”

“…[28][29][30][31][32][33][34][35]). The ejected matter from NSMs is very neutron-rich (⟨Z/A⟩ ≡ Y e ∼ 0.1).…”

Impact of Neutrino Interactions on Outflows of Neutron-Star Mergers

“…Both Newtonian simulations including neutrino (ν-) interactions (e.g. [3]) and relativistic simulations ignoring ν-interactions (e.g. [4]) have produced massive dynamical ejecta that were throughout neutron-rich enough to enable strong r-processing with fission recycling.…”

Nucleosynthesis in Supernovae, Hypernovae/Gamma-ray Bursts and Compact Binary Mergers

“…Optical and near-infrared observations of such an event, accompanying the short-duration GRB130603B have been reported [71] (see also [4]). After the first detailed nucleosynthesis predictions (following ideas of [33]) of such an event [13], many more and more sophisticated investigations have been undertaken, involving quite a number of authors [5,12,16,17,27,32,39,42,51,56,61,78] as well as the black hole accretion disk system after the formation of a central black hole [27,70,78,82].…”

“…Non-relativistic simulations (e.g. [32,61]) lead to large amounts of ejecta of the order 10 −2 M ⊙ with very small Y e -values of 0.04 and less. This causes a very strong r-process with fission cycling.…”