The role of fission on neutron star mergers and its impact on the r-process peaks

Abstract

M. Eichler, A. Arcones, A. Kelic, O. Korobkin, K. Langanke, T. Marketin, G. Martinez-Pinedo, I. Panov, T. Rauscher, S. Rosswog, C. Winteler, N. T. Zinner, and F. K. Thielemann, ???The role of fission on neutron star mergers and its impact on the r-process peaks???, in proceedings CETUP* 2015 ??? Workshop on Dark Matter, Neutrino Physics and Astrophysics PPC 2015 ??? IXth International Conference on Interconnections between Particle Physics and Cosmology. Deadwood, South Dakota, USA. 15-17 July 2015. Barbara Szczerbinska, Rouzbeh Allahverdi, Kaladi Babu, Baha Balantekin, Bhaskar Dutta, Teruki Kamon, Jason Kumar, Farinaldo Queiroz, Louis Strigari, and Rebecca Surman eds., ISBN 9780735414006. Available online at doi: http://dx.doi.org/10.1063/1.4953296. Published by AIP Publishing.The comparison between observational abundance features and those obtained from nucleosynthesis predictions of stellar evolution and/or explosion simulations can scrutinize two aspects: (a) the conditions in the astrophysical production site and (b) the quality of the nuclear physics input utilized. Here we test the abundance features of r-process nucleosynthesis calculations using four different fission fragment distribution models. Furthermore, we explore the origin of a shift in the third r-process peak position in comparison with the solar r-process abundances which has been noticed in a number of merger nucleosynthesis predictions. We show that this shift occurs during the r-process freeze-out when neutron captures and ??-decays compete and an (n,??)-(??,n) equilibrium is not maintained anymore. During this phase neutrons originate mainly from fission of material above A = 240. We also investigate the role of ??-decay half-lives from recent theoretical advances, which lead either to a smaller amount of fissioning nuclei during freeze-out or a faster (and thus earlier) release of fission neutrons, which can (partially) prevent this shift and has an impact on the second and rare-earth peak as well

Nuclear robustness of therprocess in neutron-star mergers

“…When comparing the time evolution of the third peak (from the upper to the middle to the lower panel of figure 2) one clearly notices a shift of the third peak to slightly larger mass numbers in the FRDM mass model. This is caused by neutron captures after freeze-out and confirms the findings reported in [23]. Looking at the final abundances computed for the mass models used in the present study (see Fig.…”

“…Eichler et al have confirmed the strong sensitivity on the fission yield distribution [23]. However, they succeeded to show that fission yields derived with the ablation-abrasion model code ABLA [24,25], which is based on the statistical model, cured the shortcomings in the abundance distribution above the second peak.…”

“…However, these neutrons as well as those which are produced by the decay of the fission fragments can be captured by nuclei after freezeout resulting in a slight shift of the third peak to heavier mass numbers than is observed in the solar abundances. Eichler et al [23] argue that such a shift can be avoided by faster β decays than those predicted by the Finite Range Droplet Model (FRDM) model which has been adopted in their study. In an independent study Caballero et al come to the same conclusion [26] when they compare results of NS merger r-process simulations performed with the half lives based on the FRDM model with those obtained by replacing these half lives with faster values derived by QRPA calculations on top of the energy density functional (EDF) of Fayans [27].…”

The multi-messenger picture of compact binary mergers

“…The authors suggest that a requirement for the robustness is that at the moment of freeze-out a much larger mass is in the fissioning region (nucleon number A > 250) than in the second r-process peak and above (120 < A < 180). While the results are -for fixed nuclear physics ingredients-very robust with respect to a change of the astrophysical parameters, they show a substantial sensitivity with respect to the variation of the nuclear physics input, they are particularly sensitive to the distribution of the fission products 115,155,156 which have a particular influence on the abundance pattern after the second peak. Although the abundance pattern globally agrees well with the solar system abundances, a close inspection shows in a number of studies 111, 113, 115, 138, 157-159 a slight, but systematic shift (with respect to the solar pattern) of the calculated results towards higher A.…”

“…These are attributed to "late" neutrons, e.g. from β-delayed neutron emission, but the agreement can be improved 156,160 if β-decay rates are applied that are faster than those derived from the frequently used Finite Range Droplet Model. 161 Such increased rates are actually consistent with recent experimental data for nuclei close to the r-process path 162 and recent shell model calculations.…”