Session 1WC: Explosive Nucleosynthesis in SNe and XRB as Multi-messengers

Supernovae and Nucleosynthesis

Nucleosynthesis by rapid neutron capture (the r-process) could be an important diagnostic of the explosive deep interiors of supernovae. The early appearance of r-process elements in the Galaxy, along with energetic requirements, strongly argues in favor of a supernova origin for r-process isotopes. However there is a current conundrum as to the relative contributions from various supernovae environments, e.g. MHD jets or neutrino energized winds. There are also possible contributions from failed supernovae (collapsars) leading to a black hole (BH), or the ejection of material during the mergers of neutron stars in binary systems, i.e. NS+NS or NS+BH systems. In this talk we will review the theoretical underpinnings of each possibility in the quest to deduce the relative contribution of each process. In particular, each model for r-process nucleosynthesis invariably leads to systematic discrepancies with the observed solar-system r-process abundances. For example, although the location of the abundance peaks near nuclear mass numbers $A=$130 and 195 identify an environment of rapid neutron capture near closed nuclear shells, the abundances of elements just above and below those peaks are often underproduced by more than an order of magnitude in model calculations. Similarly, most recent neutrino-driven wind simulations produce only the lighter r-process elements, while neutron-star mergers may miss the r-process peaks due to fission recycling. In this talk we demonstrate that the underproduction of elements above and below the r-process peaks can be supplemented via fission fragment distributions from the recycling of material synthesized during neutron star mergers, while the abundance peaks themselves are well reproduced in MHD jets in supernovae and collapsars. Moreover, we show that the relative contributions to the solar-system r-process yields from core-collapse supernovae and neutron star mergers required by this proposal are consistent with estimates of the relative Galactic event rates. We also describe the prospects for diagnosing the relative contributions of various sources from a measurement of the spectrum of the diffuse cosmic background of relic supernova neutrinos.   

Nuclear structure for SNe r- and neutrino processes

SNe r- and neutrino-processes are investigated based on recent advances in the studies of spin responses in nuclei. New shell-model Hamiltonians, which can well describe spin responses in nuclei with proper tensor components, are used to make accurate evaluations of reaction cross sections and rates in astrophysical processes. Nucleosyntheses in SNe r- and $\nu$-processes as well as rp-processes are discussed with these new reaction rates with improved accuracies. \begin{enumerate} \item Beta-decay rates for N=126 isotones are evaluated by shell-model calculations, and new rates are applied to study r-process nucleosynthesis in SNe's around its third peak as well as beyond the peak region up to uranium. \item $\nu$-processes for light-element synthesis in core-collapse SNe are studied with a new shell-model Hamiltonian in p-shell, SFO. Effects of MSW $\nu$-oscillations on the production yields of $^{7}$Li and $^{11}$B and sensitivity of the yield ratio on $\nu$-oscillation parameters are discussed. $\nu$-induced reactions on $^{16}$O are also studied. \item A new shell-model Hamiltonian in pf-shell, GXPF1J, is used to evaluate e-capture rates in pf-shell nuclei at stellar environments. New e-capture rates are applied to study nucleosynthesis in type-Ia supernova explosions, rp-process and X-ray bursts. \end{enumerate}   

Beta-delayed neutron emission measurements for r-process nuclei

Beta-delayed neutron- (bn-) emitters play an important, two-fold role in the stellar nucleosynthesis of heavy elements in the ``rapid neutron-capture process'' (r process). On one hand they lead to a detour of the material beta-decaying back to stability. On the other hand, the released neutrons increase the neutron-to-seed ratio, and are re-captured during the freeze-out phase and thus influence the final solar r-abundance curve. A large fraction of the isotopes for r-process nucleosynthesis are not yet experimentally accessible and are located in the ``terra incognita.'' With the next generation of fragmentation and ISOL facilities presently being built or already in operation, one of the main motivation of all projects is the investigation of very neutron-rich isotopes at and beyond the border of presently known nuclei. However, reaching more neutron-rich isotopes means also that multiple neutron-emission becomes the dominant decay mechanism. The investigation of bn-emitters has recently experienced a renaissance. I will show some recent results from a GSI campaign with the BELEN detector, and introduce the program planned for 2015/16 at RIKEN with the ``BRIKEN'' detector [1]. ``BRIKEN'' (``Beta-delayed neutron measurements at RIKEN for nuclear structure, astrophysics, and applications'') is a worldwide effort which combines 3He-neutron counters from groups in Germany, Japan, Russia, Spain, and the USA and the implantation detector AIDA from the UK to the presently largest and most efficient neutron detection setup. Planned first experiments comprise the first-time measurements of 48 b-delayed one-neutron and 24 b-delayed two-neutron emitters in the regions around doubly-magic 78Ni and 132Sn. Even some b-delayed three-neutron emitters in the heavier mass region will be tackled for the first time. \\[4pt] [1] ``The BRIKEN Neutron Detector,'' Detector Construction Proposal, approved by the RIKEN NP-PAC (2013).   

Observations of neutron-capture elements in the first stars

A considerable number of observational constraints on the nature of neutron-capture element production in the early Universe have been assembled over the past decade. For example, the neutron-capture element Sr has been detected in one of the lowest metallicity stars known, HE 1327-2326, with [Fe/H] $=$ -5.7. While only upper limits on Sr and Ba are available for the handful of other stars known with [Fe/H] \textless -4.5, the presence of Sr in HE 1327-2326 indicates that at least one channel exists for the production of elements beyond the iron peak in the most metal-poor stars. Dedicated searches for highly r-process-enhanced stars (r-II stars; [r-element/Fe] \textgreater $+$1.0) have revealed a total of some 18 such objects, roughly one-third of which exhibit the so-called ``actinide boost'' phenomenon, with Th (and sometimes U) observed at levels that are significantly higher than expected for radioactive species that have existed for \textgreater 12 Gyrs. The r-II stars occupy a relatively narrow range in metallicity, -3.3 \textless [Fe/H] \textless -2.8, which may be related to their astrophysical origin. Dedicated radial-velocity monitoring of a subset of the r-II stars has shown no preference for such stars to form as binary systems, indicating that the enhancement of their r-process elements most likely occurred due to pollution of their natal clouds. In order to better clarify the nature of the astrophysical site(s) of early neutron-capture production, and its relationship (if any) to the characteristic light-element pattern (e.g., of CNO) that is found for \textgreater 40{\%} of all stars with [Fe/H] \textless -3.5, new dedicated surveys are now being undertaken, which will be summarized in this talk.   

Photo- and neutrino-induced reactions for SNe nucleosynthesis

Neutrino-induced nuclear reactions are considered to play important roles in the dynamics of supernova explosions and in supernova nucleosynthesis. For example, neutrino-inelastic scattering off light nuclei are supposed to assist the explosion by supplying the kinetic energy to the outgoing matters. The neutrino-nucleus reactions via the neutral and charged current of the weak interaction are the key reactions in the r-process nucleosynthesis in neutrino-driven wind. To quantitatively understand those phenomena, precise data of the neutrino-nucleus reaction rates are indispensable. Recently developed secondary particle beams provide good experimental opportunities for determining the neutrino-nucleus reaction rates. A quasi-monochromatic laser Compton-scattered (LCS) photon beam is useful for studying the photonuclear reactions which are the direct analogue of the neutrino inelastic scatterings caused by the weak neutral current. On the other hand, real neutrino beams are ideal tools to directly measure the absolute neutrino-nucleus reaction rates. Another interesting probe will be the nuclear muon-capture reaction, because it can be applied for measurement of the targets with very small quantities thanks to its large capture probability. In this talk recent progress in ongoing experiments with LCS gamma-rays and muon beams will be presented. A new plan for direct measurement of the neutrino-nucleus reactions with an accelerator-driven neutrino beam will be also discussed.   

Mass Measurement for R-process Nuclei at RIBF

Precise mass measurements of extremely rare radioactive nuclei (RI) are critical in revealing nature of the r-process nucleosynthesis that is considered to be an origin of elements heavier than iron. Unprecedentedly high intensity RI beams produced at RIBF provide us a good chance to make a direct mass measurement of the r-process nuclei. At RIKEN, two advanced devices for the mass measurement have been developed: Rare RI Ring and MR-TOF coupled with SLOWRI. In the workshop, I will discuss a strategic plan of mass measurements of r-process nuclei, with the two devices, at RIBF.