Narrow Results

Earlier weak rate calculations relied on the Brink–Axel hypothesis (BAH), but the assumption requires validation against microscopic calculations for reliable nucleosynthesis modeling. We investigate the effectiveness of BAH in computing stellar weak rates of ${\beta }^{-}$–decay and electron capture interactions in two sets of heavy neutron-rich nuclei: fp- and fpg-shell nuclei ($26\le Z\le 31$, $55\le A\le 82$) for post-silicon burning phases, and waiting-point nuclei ($24\le Z\le 81$, $74\le A\le 207$) for r-process. The impact of BAH is evaluated for allowed rates in the first set, and for unique first-forbidden and allowed rates in the second set. Our calculations used the proton–neutron quasi-particle random-phase approximation (pn-QRPA) model with two strength function prescriptions: one using state-by-state calculations and the other invoking the BAH, within stellar densities $(10^7\text{–}10^{11})$g/cm³ and temperatures $(5\text{–}30)$GK of post-silicon burning phase of massive stars. Our study reveals significant deviations between BAH-invoked and microscopic ${\beta }^{-}$–decay rates, with overestimation up to three orders of magnitude in allowed rates for fp/fpg-shell nuclei and for waiting-point nuclei exhibiting a stark difference in U1F rates up to four–five orders of magnitude, contrasting with relatively small deviations by up to an order of magnitude in electron capture rates. Our results suggest that application of BAH may lead to unrealistic weak stellar allowed (forbidden) ${\beta }^{-}$-decay rates.

The exact physical conditions generating the abundances of r-elements in environments such as supernovae explosions are still under debate. We evaluated the characteristics expected for the neutrino wind in the proposed model of type-II supernova driven by conversion of nuclear matter to strange matter. Neutrinos will change the final abundance of elements after freeze out of r-process nucleosynthesis, specially those close to mass peaks.

Neutron star mergers have been long considered as promising sites of heavy $r$-process nucleosynthesis. We overview the observational evidence supporting this scenario including: the total amount of $r$-process elements in the galaxy, extreme metal-poor stars, geological radioactive elemental abundances, dwarf galaxies and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and Virgo observatories discovered a gravitational-wave signal of a neutron star merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM) counterparts. The ultra-violet, optical and near infrared (n/R) observations point to $r$-process elements that have been synthesized in the merger ejecta. The rate and ejected mass inferred from GW170817 and the EM counterparts are consistent with other observations. We however, find that, within the simple one zone chemical evolution models (based on merger rates with reasonable delay time distributions as expected from evolutionary models, or from observations of sGRBs), it is difficult to reconcile the current observations of the Eu abundance history of galactic stars for [Fe/H] $\gtrsim -1$. This implies that to account for the role of mergers in the galactic chemical evolution, we need a galactic model with multiple populations that have different spatial distributions and/or varying formation rates.

Neutrino-induced reactions are a basic ingredient in astrophysical processes like star evolution. The existence of neutrino oscillations affects the rate of nuclear electroweak decays which participates in the chain of events that determines the fate of the star. Among the processes of interest, the production of heavy elements in core-collapse supernovae is strongly dependent upon neutrino properties, like the mixing between different species of neutrinos. In this work, we study the effects of neutrino oscillations upon the electron fraction as a function of the neutrino mixing parameters, for two schemes: the $1+1$-scheme (one active neutrino and one sterile neutrino) and the $2+1$-scheme (two active neutrinos and one sterile neutrino). We have performed this analysis considering a core-collapse supernovae and determined the physical conditions needed to activate the nuclear reaction chains involved in the r-process. We found that the interactions of the neutrinos with matter and among themselves and the initial amount of sterile neutrinos in the neutrino-sphere might change the electron fraction, therefore affecting the onset of the r-process. We have set constrains on the active-sterile neutrino mixing parameters. They are the square-mass-difference $\mathrm{\Delta }{m}_{14}^2$, the mixing angle ${sin}^{2}2{𝜃}_{14}$, and the hindrance factor ${\xi }_s$ for the occupation of sterile neutrinos. The calculations have been performed for different values of $X_{\alpha }$, which is the fraction of $\alpha$-particles. For $X_{\alpha }=0$ the r-process is taking place if $\mathrm{\Delta }{m}_{14}^2\ge 2{\mathrm{\text{eV}}}^2$, ${sin}^{2}2{𝜃}_{14}<0.8$ and ${\xi }_s<0.5$. For larger values of $X_{\alpha }$ the region of parameters is strongly reduced. The present results are compared to results available in the literature.

The low-lying dipole and quadrupole states in neutron rich nuclei are studied within the fully self-consistent relativistic quasiparticle random-phase approximation (RQRPA), formulated in the canonical basis of the Relativistic Hartree–Bogoliubov model (RHB), which is extended to include the density dependent interactions. In heavier nuclei, the low-lying E1 excited state is identified as a pygmy dipole resonance (PDR), i.e. as a collective mode of excess neutrons oscillating against a proton–neutron core. Isotopic dependence of the PDR is characterized by a crossing between the PDR and one-neutron separation energies. Already at moderate proton–neutron asymmetry the PDR peak is calculated above the neutron emission threshold, indicating important implications for the observation of the PDR in (γ, γ′) scattering, and on the theoretical predictions of the radiative neutron capture rates in neutron-rich nuclei. In addition, a novel method is suggested for determining the neutron skin of nuclei, based on measurement of excitation energies of the Gamow–Teller resonance relative to the isobaric analog state.

Electric dipole strength below the particle emission threshold both in stable nuclei and short-lived isotopes has received increasing interest due to its astrophysical impact. In analogy to the giant dipole resonance, this strength is commonly referred to as pygmy resonance. Coulomb dissociation of neutron-rich unstable isotopes and nuclear resonance fluorescence photon scattering have begun to provide systematic data on electric dipole strength in various isotope chains. We review the present state of the art of theoretical approaches and point out some open problems. We emphasize the necessity of a simultaneous theoretical treatment of the nucleon separation energies and the energetically low-lying dipole strength because the presently available data do not exclude a non-collective nature of the pygmy strength.

Neutron rich nuclei has been studied with a new phenomenological mass formula. Predictions of different mass formulas for the location of the neutron drip line are compared with those from the present calculation. The implications of the new mass formula for r-process nucleosynthesis are discussed. It is found that though the neutron drip line obtained from this formula differs substantially from other formulas, the r-process abundance upto mass 200 are unlikely to be significantly different. The errors inherent in the mass formula are found to play an insignificant role beyond mass A = 80.

A possible mechanism for the formation of heavy-mass elements in supernovae is the rapid neutron-capture-mechanism ($r$-process). It depends upon the electron-fraction $Y_e$, a quantity which is determined by beta-decay-rates. In this paper, we focus on the calculation of electroweak decay-rates in presence of massive neutrinos. The resulting expressions are then used to calculate nuclear reactions entering the rapid-neutron capture. We fix the astrophysical parameters to the case of a core-collapse supernova. The neutrino sector includes a mass scheme and mixing angles for active neutrinos, and also by including the mixing between active and sterile neutrinos. The results of the calculations show that the predicted abundances of heavy-mass nuclei are indeed affected by the neutrino mixing.

This paper examines nucleosynthesis in a low-mass neutron star crust that loses mass due to accretion in a close binary system and reaching a hydrodynamically unstable configuration explodes. The r-process proceeds mainly in the inner crust. Nucleosynthesis in the outer crust is an explosive process with a sharp increase in temperature caused by an outward-propagating shockwave. The number of heavy elements produced in a low-mass neutron star crust during the explosion is approximately 0.041 $M_{\odot }$, which exceeds the number of heavy elements ejected as jets in the neutron star merger scenario.

R-processes are a new type of discrete stationary stochastic process. In this paper, we investigate the underlying structure of two special classes of r-processes. For each class, we will prove precisely when the associated multidigraph is perfect.

In the present report the influence of fission mass distribution model on the final abundance of heavy elements in the matter, ejected during neutron star mergers is discussed. Different models of fission fragment mass distribution for exotic nuclei were applied to the r-process, calculated for the scenario of neutron star mergers, and the significant influence of fission fragment mass distribution model on the abundance of heavy nuclei was shown. We have found that fission fragment mass distribution model is most important for fissioning nuclei with A > 260 from the region near to neutron drip-line and for superheavy nuclei. It was shown that tri-partition or ternary fission, the role of which can strongly rise in neutron rich hot dense matter, strongly affected on the abundance of heavy nuclei.

Nuclear astrophysics is a active, rapidly developing field that addresses fundamental scientific questions. Advances in astronomy have to be matched with advances in nuclear physics to address these questions in the future. I discuss some of the major open questions and future directions, using as examples supernovae, the rapid neutron capture process, and processes on the surface of accreting neutron stars. In all cases nuclear physics is needed to interpret observations and to guide and constrain theoretical models. While a wide range of experimental facilities is required, one of the major challenges for the future is the study of unstable nuclei that play a central role in many of the major open questions. Therefore, an advanced radioactive beam facility such as the proposed Rare Isotope Accelerator (RIA) is of particular importance for the future of this field.

In present report we discuss the dependence of Galaxy age, which can be defined on the basis of galactic nucleosynthesis by method of thorium and uranium isotopic relations, and scenario conditions of heavy nuclei nucleosynthesis. It was shown that Galaxy age strongly depend on the r-process scenario, its nuclear data input and on parameters of galactic nucleosynthesis as well. The influence of additional nucleosynthesis spike close to proto-solar cloud was defined. The tolerance range for nuclei-cosmochronometers abundances ratios and parameters of galactic nucleosynthesis are discussed.

We have explored the Eu production in the Milky Way by means of a very detailed chemical evolution model. In particular, we have assumed that Eu is formed in merging neutron star (or neutron star-black hole) binaries as well as in Type II supernovae. We have tested the effects of several important parameters influencing the production of Eu during the merging of two neutron stars, such as (i) the time-scale of coalescence, (ii) the Eu yields and (iii) the range of initial masses for the progenitors of the neutron stars. The yields of Eu from Type II supernovae are very uncertain, more than those from coalescing neutron stars, so we have explored several possibilities. We have compared our model results with the observed rate of coalescence of neutron stars, the solar Eu abundance, the [Eu/Fe] versus [Fe/H] relation in the solar vicinity. Our main results can be summarized as follows: (i) neutron star mergers can be entirely responsible for the production of Eu in the Galaxy if the coalescence time-scale is no longer than 1 Myr for the bulk of binary systems, the Eu yield is around 3 · 10⁻⁶M_⊙ and the mass range of progenitors of neutron stars is 9-50 M_⊙; (ii) both Type II supernovae and merging neutron stars can produce the right amount of Eu if the neutron star mergers produce 2·10⁻⁶M_⊙ per system and Type II supernovae, with progenitors in the range 20-50 M_⊙, produce yields of Eu of the order of 10⁻⁸ − 10⁻⁹M_⊙; iv) The observed spread in the [Eu/Fe] ratio in Milky Way halo stars can be reproduced if an inhomogeneous stochastic model is adopted for the early phases of the halo formation.

The age of the Universe is one of the most important physical quantities in cosmology and it can be determined with the r-process nucleochronometer. Based on the classical r-process model, the r-process abundance patterns and various nuclear chronometers in the metal-poor halo stars are investigated by employing the newly developed nuclear mass models. It is found that the un-certainty of Th/Eu chronometer caused by nuclear mass uncertainties is about 4 Gyr, while the uncertainty of Th/Hf chronometer is so large that it should be taken with caution. With the Th/Eu chronometer, the age of the metal-poor stars CS 31082-001 is determined as 16.3±7.1 Gyr, which agrees well with the results derived from Th/U chronometer.

An attempt has been made to identify the most critical nuclei for measurements at the new and developing facilities around the world for a better understanding of the rapid neutron capture process in terms of the input nuclear physics. A series of nucleosynthesis simulations have been carried out using some standard astrophysical conditions and several nuclear mass models including the FRDM (Finite Range Droplet Model) of Moller et al.,^1,2 the Duflo-Zuker mass model,³ the quenched ETFSIQ of Pearson et al.,⁴ and the F-spin model of Teymurazyan et al.⁵ The most critical nuclei were determined from their impacts on the resulting r-process abundances. Impact was determined in two ways; by their relative and absolute effect on the overall elemental abundance patterns. In all the models considered, it was found uniformly that nuclei near the closed shells were the ones with the greatest impact and therefore the most critical nuclei to measure.

New shell model Hamiltonians, which give successful description of spin responses in nuclei, are applied to astrophysical processes in stars induced by nuclear weak interactions. Neutrino-nucleus reactions in supernova explosions, electron capture reactions in stellar core-collapse process and β-decays in r-process are discussed.

Based on the (n, γ) ⇌ (γ, n) equilibrium, the neutron density and temperature conditions required for the r-process are constrained with updated nuclear masses. It is found that the uncertainty of determined neutron density and temperature ranges can be greatly minimized when mass values tabulated in the latest Atomic Mass Evaluation AME2011-preview are employed.

The roles of nuclear weak processes in stars are discussed. Neutrino-nucleus reactions on ¹²C, ⁵⁶Fe and ⁴⁰Ar are studied with new shell-model Hamiltonians. New cross sections, which give good account of experimental data, are applied to nucleosynthesis of light elements in supernova explosions. Effects of ν-oscillations are investigated, and the abundance ratio of ⁷Li/¹¹B is pointed out to be sensitive to the ν mass hierarchy. Then, e-capture and β-decay rates in sd-shell nuclei are evaluated at stellar environments, and applied to study cooling of O-Ne-Mg core stars by nuclear URCA processes. The fate of 8-10 M_❿ stars is sensitive to the cooling of the core. Finally, β-decay half-lives of r-process waiting-point nuclei with N=126 are evaluated by shell-model calculations taking into account both the Gamow–Teller and first-forbidden transitions. The half-lives obtained are short compared with standard FRDM values.

The development of a high intensity ²³⁸U beam at the Radioactive Isotope Beam Factory (RIBF) has opened a new opportunity to explore exotic regions of the nuclear chart that were not accessible before. Along with beam development, the installation of the high efficiency γ-detector EURICA has made β-decay spectroscopy measurements of these regions possible, and a large international effort named the EURICA project has been launched to take advantage of this new opportunity.