Production of the entire range ofr-process nuclides by black hole accretion disc outflows from neutron star mergers

Abstract

We consider r-process nucleosynthesis in outflows from black hole accretion discs formed in double neutron star and neutron star -black hole mergers. These outflows, powered by angular momentum transport processes and nuclear recombination, represent an important -and in some cases dominant -contribution to the total mass ejected by the merger. Here we calculate the nucleosynthesis yields from disc outflows using thermodynamic trajectories from hydrodynamic simulations, coupled to a nuclear reaction network. We find that outflows produce a robust abundance pattern around the second r-process peak (mass number A ∼ 130), independent of model parameters, with significant production of A < 130 nuclei. This implies that dynamical ejecta with high electron fraction may not be required to explain the observed abundances of r-process elements in metal poor stars. Disc outflows reach the third peak (A ∼ 195) in most of our simulations, although the amounts produced depend sensitively on the disc viscosity, initial mass or entropy of the torus, and nuclear physics inputs. Some of our models produce an abundance spike at A = 132 that is absent in the Solar system r-process distribution. The spike arises from convection in the disc and depends on the treatment of nuclear heating in the simulations. We conclude that disc outflows provide an importantand perhaps dominant -contribution to the r-process yields of compact binary mergers, and hence must be included when assessing the contribution of these systems to the inventory of r-process elements in the Galaxy.

Impact of Neutrino Interactions on Outflows of Neutron-Star Mergers

“…2) is dominated by intermediate mass elements with 90 < ∼ A < ∼ 140. Heavier elements up to A ∼ 240 are produced as well, but compared to the lighter elements their abundances vary strongly with the torus and BH masses, and the employed viscosity treatment (which is also confirmed by [9]). Our results, in addition to those by [6,10,11] who investigated NS-torus remnants, indicate that merger remnants can throw out ejecta with comparable masses but rather different abundance patterns with respect to the dynamical ejecta.…”

“…[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).…”

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].…”

“…4. Black hole accretion disks are the final stage of neutron star mergers and can also provide the lower mass r-process elements [27,82]. They will also occur for the collapse of massive stars resulting in lGRBs/Hypernovae, but are not expected to produce heavy r-elements (unless special neutrino properties permit so, [69]).…”