M. Terasawa

New Nuclear Reaction Flow during r-Process Nucleosynthesis in Supernovae: Critical Role of Light, Neutron-rich Nuclei

We study the role of light, neutron-rich nuclei during r-process nucleosynthesis in supernovae. Most previous studies of the r-process have concentrated on the reaction flow of heavy, unstable nuclei. Although the nuclear reaction network includes a few thousand heavy nuclei, only limited reaction flow through light nuclei near the stability line has been used in those studies. However, in a viable scenario of the r-process in neutrino-driven winds, the initial condition is a high-entropy hot plasma consisting of neutrons, protons, and electron-positron pairs experiencing an intense flux of neutrinos. In such environments, light nuclei, as well as heavy nuclei, are expected to play important roles in the production of seed nuclei and r-process elements. Thus, we have extended our fully implicit nuclear reaction network so that it includes all nuclei up to the neutron-drip line for Z ≤ 10, in addition to a larger network for Z ≥ 10. In the present nucleosynthesis study, we utilize a wind model of massive Type II supernova explosions to study the effects of this extended network. We find that a new nuclear reaction flow path opens in the very light, neutron-rich region. This new nuclear reaction flow can change the final heavy-element abundances by as much as an order of magnitude.

r-Process in Prompt Supernova Explosions Revisited

We reanalyze r-process nucleosynthesis in the neutron-rich ejecta from a prompt supernova explosion of a low-mass (11 M?) progenitor. Although it has not yet been established that a prompt explosion can occur, it is not yet ruled out as a possibility for low-mass supernova progenitors. Moreover, there is mounting evidence that a new r-process site may be required. Hence, we assume that a prompt explosion can occur and make a study of r-process nucleosynthesis in the supernova ejecta. To achieve a prompt explosion we have performed a general relativistic hydrodynamic simulation of adiabatic collapse and bounce using a relativistic nuclear-matter equation of state. The electron fraction Ye during the collapse was fixed at the initial-model value. The size of the inner collapsing core was then large enough to enable a prompt explosion to occur in the hydrodynamic calculation. Adopting the calculated trajectories of promptly ejected material, we explicitly computed the burst of neutronization due to electron captures on free protons in the photodissociated ejecta after the passage of the shock. The thermal and compositional evolution of the resulting neutron-rich ejecta originating from near the surface of the proto-neutron star was obtained. These were used in nuclear reaction network calculations to evaluate the products of r-process nucleosynthesis. We find that, unlike earlier studies of nucleosynthesis in prompt supernovae, the amount of r-process material ejected per supernova is quite consistent with observed Galactic r-process abundances. Furthermore, the computed r-process abundances are in good agreement with solar abundances of r-process elements for A > 100. This suggests that prompt supernovae are still viable r-process sites. Such events may be responsible for the abundances of the heaviest r-process nuclei.

Nucleosynthesis of Light Elements and Heavy r-Process Elements through the ν-Process in Supernova Explosions

We study the nucleosynthesis of the light elements 7Li and 11B and the r-process elements in Type II supernovae from the point of view of supernova neutrinos and Galactic chemical evolution. We investigate the influence of the luminosity and average energy (temperature) of supernova neutrinos on these two nucleosynthesis processes. Common models of the neutrino luminosity, which is parameterized by the total energy Eν and decay time τν, and neutrino temperature are adopted to understand both processes. We adopt the model of the supernova explosion of a 16.2 M☉ star, which corresponds to SN 1987A, and calculate the nucleosynthesis of the light elements by postprocessing. We find that the ejected masses of 7Li and 11B are roughly proportional to the total neutrino energy and are weakly dependent on the decay time of the neutrino luminosity. As for the r-process nucleosynthesis, we adopt the same models of the neutrino luminosity in the neutrino-driven wind models of a 1.4 M☉ neutron star. We find that the r-process nucleosynthesis is affected through the peak neutrino luminosity, which depends on Eν/τν. The observed r-process abundance pattern is better reproduced at a low peak neutrino luminosity. We also discuss the unresolved problem of the overproduction of 11B in the Galactic chemical evolution of the light elements. We first identify that the ejected mass of 11B is a factor of 2.5-5.5 overproduced in Type II supernovae when one adopts neutrino parameters similar to those in previous studies, i.e., Eν = 3.0 × 1053 ergs, τν = 3 s, and a neutrino temperature T = T = 8.0 MeV/k. We have to assume Eν ≤ 1.2 × 1053 ergs to avoid the overproduction of 11B, which is too small to accept in comparison to the 3.0 × 1053 ergs deduced from the observation of SN 1987A. We here propose to reduce the temperatures of νμ,τ and μ,τ to 6.0 MeV/k in a model with Eν ~ 3.0 × 1053 ergs and τν ~ 9 s. This modification of the neutrino temperature is shown to resolve the overproduction problem of 11B while still keeping a successful r-process abundance pattern.

r-process nucleosynthesis in neutrino-driven winds from a typical neutron star with M = 1.4 M⊙

We study the effects of the outer boundary conditions in neutrino-driven winds on the r-process nucleosynthesis. We perform numerical simulations of hydrodynamics of neutrino-driven winds and nuclear reaction network calculations of the r-process. As an outer boundary condition of hydrodynamic calculations, we set a pressure upon the outermost layer of the wind, which is approaching toward the shock wall. Varying the boundary pressure, we obtain various asymptotic thermal temperature of expanding material in the neutrino-driven winds for resulting nucleosynthesis. We find that the asymptotic temperature slightly lower than those used in the previous studies of the neutrino-driven winds can lead to a successful r-process abundance pattern, which is in a reasonable agreement with the solar system r-process abundance pattern even for the typical proto-neutron star mass Mns ~ 1.4 Msun. A slightly lower asymptotic temperature reduces the charged particle reaction rates and the resulting amount of seed elements and lead to a high neutron-to-seed ratio for successful r-process. This is a new idea which is different from the previous models of neutrino-driven winds from very massive (Mns ~ 2.0 Msun) and compact (Rns ~ 10 km) neutron star to get a short expansion time and a high entropy for a successful r-process abundance pattern. Although such a large mass is sometimes criticized from observational facts on a neutron star mass, we dissolve this criticism by reconsidering the boundary condition of the wind. We also explore the relation between the boundary condition and neutron star mass, which is related to the progenitor mass, for successful r-process.

Hydrodynamical study of neutrino driven wind as an r process site

We discuss the neutrino-driven wind from a proto-neutron star based on general-relat ivistic hydrodynamical simulations. We examine the properties of the neutrino-driven wind to explore the possibility of r-process nucleosynthesis. Numerical simulations involving neutrino heating and cooling processes were performed with the assumption of a constant neutrino luminosity by using realistic profiles of the protoneutron star (PNS) as well as simplified models. The dependence on the mass of PNS and the neutrino luminosity is presented systematically. Comparisons with analytic treatments in the previous studies are also given. In the cases with the realistic PNS, we have found that the entropy per baryon and the expansion time scale are neither high nor short enough for the r-process within the current assumptions. On the other hand, we have also found that the expansion time scale obtained by hydrodynamica l simulations is systematically shorter than that in the analytic solutions due to our proper treatment of the equation of state. This fact might lead to an increase in the neutron-to-seed ratio, which is suitable for the r-process in a neutrino-drive n wind. Indeed, in the case of massive and compact proto-neutron stars with high neutrino luminosities, the expansion time scale is found to be sufficiently short in hydrodynamica l simulations, and the r-process elements up to A ~ 200 are produced in the r-process network calculation.

NEUTRINO EFFECTS BEFORE, DURING, AND AFTER THE FREEZEOUT OF THE r-PROCESS

We study the effects of neutrino interactions before, during, and after the operation of the r-process in the context of a relativistic hydrodynamic neutrino-energized wind model that includes a Boltzmann solver for the dominant neutrino transport. We also employ newly available charged- and neutral-current interaction rates. Studies are made in a model with a short dynamical timescale that gives a fair reproduction of the solar system r-process abundance curve and hence could be a fair approximation to the true supernova environment. We confirm that charged- and neutral-current interactions can have specific unique effects on the final abundances. Early on, charged-current interactions determine the electron fraction, while later on, neutrino-induced neutron emission can continue to provide a slight neutron exposure even after the freezeout of the r-process. We propose two new potentially observable effects that derive from the use of a more realistic hydrodynamic model and/or new neutrino interaction rates. One is an enhanced odd-even effect in the final abundances, and the other is an enhancement of light A = 68-76 nuclei in the final abundances. These effects are consistent with some recent observations of r-process abundances in metal-poor halo stars and might help to identify the neutrino fluxes present near the freezeout of the r-process.

Direct measurements of (α, n) and (p, n) reaction cross sections using light neutron-rich radioactive nuclear beams

Abstract A project to acquire directly nuclear cross sections of (α, n) and (p, n) reactions using low energy light neutron-rich radioactive nuclear beams (RNB) is in progress at JAERI-Tandem facility. The method of the production of neutron-rich RNB, an improvement of a gas counter (MSTPC) in order to work effectively under the high RNB-injection rate, and the feasibility of our experiments are discussed.

Measurement of the 16N(α, n) reaction cross section

Abstract In order to study the starting point of the r-process, the 16 N(α, n) cross section has been measured directly at the energy region of E cm = 1.5 – 4.0 MeV, corresponding to the Gamow peak energy T 9 = 2 – 6.

Study of Astrophysical (α, n) and (p, n) Reactions on Light Neutron‐Rich Nuclei by Means of Low‐Energy RNB

A systematic study of astrophysical reaction rates of (α, n) and (p, n) reactions on light neutron‐rich nuclei by using low‐energy radioactive nuclear beam is in progress at the tandem facility of Japan Atomic Energy Research Institute. Exclusive measurements of 8Li(α, n)11B and 16N(α, n)19F reaction cross sections have been performed successfully. Their excitation functions together with the experimental method are presented.

The r-process in neutrino-driven winds from a typical neutron star with M = 1.4M⊙

Abstract We study the effects of the outer boundary conditions in neutrino-driven winds on the r-process nucleosynthesis. Varying the boundary condition, we obtain various asymptotic thermal temperature of expanding material. As a result, we find that the slightly lower asymptotic temperature than those in the previous studies of the neutrino-driven winds can lead to a successful r-process even for the typical proto-neutron star mass MNS 1.4M⊙. We also discuss the relation between the boundary condition and neutron star mass, which is related to the progenitor mass, for the successful r-process