Nuclear robustness of therprocess in neutron-star mergers

Abstract

We have performed r-process calculations for matter ejected dynamically in neutron star mergers based on a complete set of trajectories from a three-dimensional relativistic smoothed particle hydrodynamic simulation with a total ejected mass of ∼ 1.7 × 10 −3 M . Our calculations consider an extended nuclear network, including spontaneous, β-and neutron-induced fission and adopting fission yield distributions from the ABLA code. In particular we have studied the sensitivity of the r-process abundances to nuclear masses by using different models. Most of the trajectories, corresponding to 90% of the ejected mass, follow a relatively slow expansion allowing for all neutrons to be captured. The resulting abundances are very similar to each other and reproduce the general features of the observed r-process abundance (the second and third peaks, the rare-earth peak and the lead peak) for all mass models as they are mainly determined by the fission yields. We find distinct differences in the predictions of the mass models at and just above the third peak, which can be traced back to different predictions of neutron separation energies for r-process nuclei around neutron number N = 130. In all simulations, we find that the second peak around A ∼ 130 is produced by the fission yields of the material that piles up in nuclei with A 250 due to the substantially longer beta-decay half-lives found in this region. The third peak around A ∼ 195 is generated in a competition between neutron captures and β decays during r-process freeze-out. The remaining trajectories, which contribute 10% by mass to the total integrated abundances, follow such a fast expansion that the r process does not use all the neutrons. This also leads to a larger variation of abundances among trajectories as fission does not dominate the r-process dynamics. The resulting abundances are in between those associated to the r and s processes. The total integrated abundances are dominated by contributions from the slow abundances and hence reproduce the general features of the observed r-process abundances. We find that at timescales of weeks relevant for kilonova light curve calculations, the abundance of actinides is larger than the one of lanthanides. This means that actinides can be even more important than lanthanides to determine the photon opacities under kilonova conditions. Moreover, we confirm that the amount of unused neutrons may be large enough to give rise to another observational signature powered by their decay.

THE ROLE OF FISSION IN NEUTRON STAR MERGERS AND ITS IMPACT ON THEr-PROCESS PEAKS

“…If the neutron density is still sufficiently high, several neutron captures after freeze-out can shift the peak. It is possible that for mass models not utilized in this study, which have the third peak shifted to lower masses in (n,γ)-(γ,n) equilibrium (see e.g., Mendoza-Temis et al 2014), the final neutron captures shift the peak to its correct position.…”

“…For recent results see also Just et al (2014), Mendoza-Temis et al (2014), and Wanajo et al (2014). The combination of very low-Y e material and rapid expansion of the ejecta guarantees the occurrence of a strong r-process.…”

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

“…Shorter half-lives [38] of heavies release neutrons from fission earlier, when n, γ − γ, n equilibrium is still in place and can avoid or strongly reduce the late shift of the 3rd peak [12] (see Fig.3a). This effect is also seen with the HFB mass model (see also [18]), while Mendoza-Temis [42] analyzed additional nuclear uncertainties and showed especially an improvement with the Duflo-Zuker mass model. Investigations [65], based on quite different fission barrier predictions, which introduce fission in the r-process only above A=300, do not reproduce the 2nd r-process peak in neutron star 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].…”

“…This causes a very strong r-process with fission cycling. The utilized mass model, beta-decay half-lives, fission barriers, and fission yield prescriptions [12,18,29,38,42,55,65,78] have a strong effect on the final abundance distribution. While during the r-process (when reactions are still in n, γ − γ, n equilibrium) the 2nd and 3rd r-process peak are exactly at the right position.…”