Comprehensive nucleosynthesis analysis for ejecta of compact binary mergers

Abstract

We present the first comprehensive study of r-process element nucleosynthesis in the ejecta of compact binary mergers (CBMs) and their relic black-hole (BH)-torus systems. The evolution of the BH-accretion tori is simulated for seconds with a Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment closure scheme for the transport of electron neutrinos and antineutrinos. The investigated cases are guided by relativistic double neutron star (NS-NS) and NS-BH merger models, producing ∼3-6 M BHs with rotation parameters of A BH ∼ 0.8 and tori of 0.03-0.3 M . Our nucleosynthesis analysis includes the dynamical (prompt) ejecta expelled during the CBM phase and the neutrino and viscously driven outflows of the relic BH-torus systems. While typically ∼20-25% of the initial accretion-torus mass are lost by viscously driven outflows, neutrino-powered winds contribute at most another ∼1%, but neutrino heating enhances the viscous ejecta significantly. Since BH-torus ejecta possess a wide distribution of electron fractions (0.1-0.6) and entropies, they produce heavy elements from A ∼ 80 up to the actinides, with relative contributions of A > ∼ 130 nuclei being subdominant and sensitively dependent on BH and torus masses and the exact treatment of shear viscosity. The combined ejecta of CBM and BH-torus phases can reproduce the solar abundances amazingly well for A > ∼ 90. Varying contributions of the torus ejecta might account for observed variations of lighter elements with 40 Z 56 relative to heavier ones, and a considerable reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly asymmetric NS-BH mergers, might explain the composition of heavy-element deficient stars.

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

“…These can contribute also the lighter r-process nuclei. In a similar way ejecta of the black hole accretion disk, also powered by neutrinos (and viscous disk heating), provide the additional abundance component of light r-process nuclei [18,27,78,82].…”

“…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].…”

Impact of Neutrino Interactions on Outflows of Neutron-Star Mergers

“…[5]). However, it was recently realized that the nucleosynthesis signature of NS mergers is more complex when taking into account ν-interactions in relativistic NS-NS merger simulations and when the ejecta No e ± -capture, no !-interactions for " < " from the merger remnant are consistently included [1,[6][7][8][9][10]. In contrast to NS-BH mergers, in NS-NS mergers the large pressure gradient that is generated by the collision shock building up between both NSs causes a sizable amount of material to become unbound in addition to possible tidal-tail ejecta from the disrupted NS(s).…”

“…In [8], we followed the long-term evolution of post-merger BH-torus systems resulting from NS-BH and NS-NS mergers taking into account ν-cooling and ν-heating by means of energy-dependent, multidimensional neutrino transport while employing approximate treatments for the general relativistic gravitation of the BH and for the angular momentum transport. As a generic outcome of these models, a significant fraction of ∼ 10−20 % of the initial torus mass (with the latter being of the order of ∼ 0.01 − 0.1 M ⊙ ) becomes unbound, mainly as a result of viscous heating and angular momentum transport.…”

“…Likewise to [8] we employed the ALCAR-AENUS code [13] but upgraded its Newtonian hydrodynamics module to a special relativistic version. In contrast to previous studies we could then self-consistently follow the combined neutrino-hydrodynamic evolution of the BH-torus system, the eventual launch of the jet in response to heating by νν-annihilation, and the jet propagation through the envelope of dynamical ejecta produced during the binary merger.…”