Context.
Fifteen abnormally phosphorus-rich stars were recently discovered. Their peculiar surface abundance distribution is challenging our present stellar nucleosynthesis theories, because none of ...the standard thermal nucleosynthesis models are found to explain the observed patterns.
Aims.
This paper presents an exploration of the extent to which an irradiation process resulting from the interaction of some target material with energetic protons and/or
α
-particles can by itself be at the origin of the chemical pollution observed at the surface of P-rich stars.
Methods.
In light of our lack of knowledge of the characteristics of the accelerated particles that could be held responsible for this nuclear process, a purely parametric site-independent approach is followed, with the proton and
α
-particle flux amplitude, energy distribution, and fluence taken as free parameters. The irradiated material is assumed to be made either of CNO elements in solar ratio or pure C.
Results.
Such an irradiation process with energies of no more than a few MeV per nucleon is shown to give rise to rich nucleosynthesis, including significant production of P, as well as
Z
> 30 heavy elements in relative abundance similar to what the slow neutron-capture process traditionally produced.
Conclusions.
The final composition obtained by mixing such a non-thermal nucleosynthesis by accelerated particles with nuclearly unaffected material is found to reproduce fairly well the global surface composition of P-rich stars, except for a few species like Al, Si, or Ba.
The rapid neutron-capture process, or r-process, is known to be of fundamental importance for explaining the origin of approximately half of the
A
> 60 stable nuclei observed in nature. Despite ...important efforts, the astrophysical site of the r-process remains unidentified. Here we study r-process nucleosynthesis in a material that is dynamically ejected by tidal and pressure forces during the merging of binary neutron stars. r-process nucleosynthesis during the decompression is known to be largely insensitive to the detailed astrophysical conditions because of efficient fission recycling, producing a composition that closely follows the solar r-abundance distribution for nuclei with mass numbers A > 140. Due to the important role played by fission in such a scenario, the impact of fission is carefully analyzed. We consider different state-of-the-art global models for the determination of the fission paths, nuclear level densities at the fission saddle points and fission fragment distributions. Based on such models, the sensitivity of the calculated r-process abundance distribution is studied. The fission path is found to strongly affect the region of heavy nuclei responsible for the fission recycling, while the fission fragment distribution of nuclei along the
A
≃ 278 isobars defines the abundance pattern of nuclei produced in the 110 ≲
A
≲ 170 region. The late capture of prompt fission neutrons is also shown to affect the abundance distribution, and in particular the shape of the third r-process peak around
A
≃ 195.
We construct two new Hartree–Fock–Bogoliubov mass models, labeled HFB-28, HFB-29, which in addition to the generalized Skyrme form containing t4 and t5 terms, also include now a modified spin–orbit ...force. This alternative spin–orbit force allows for an unconventional isospin and/or density dependence relative to the traditional form included in Skyrme functionals. The new forces underlying these models are fitted to essentially all mass data and at the same time to a realistic equation of state of neutron matter. The inclusion of the modified spin–orbit terms allows us to reduce the rms deviation with respect to all the 2353 known masses with Z,N≥8 by 20 keV, leading to a final model error of 0.52 MeV. It is shown that the newly optimized spin–orbit forces do not conform with the one deduced from the relativistic mean field theory, and consequently that the relativistic spin–orbit force might not be optimum to reproduce experimental masses. The spin–orbit splittings are shown to be reduced and in better agreement with empirical values when including a density-dependent form of the spin–orbit interaction. However, the new mass models with such modified spin–orbit terms still fail to reproduce the kink seen in the isotopic shift of the K or Pb charge radii around the neutron magic numbers, despite the fact that such generalizations of the spin–orbit terms were also introduced to improve the description of the isotopic shifts. Shell effects, in particular far away from stability, are shown to remain unaffected by the new spin–orbit terms, except at the N=184 magic number.
A century ago, nuclear physics entered astrophysics, giving birth to a new field of science referred to as “Nuclear Astrophysics”. With time, it developed at an impressive pace into a vastly inter- ...and multidisciplinary field bringing into its wake not only astronomy and cosmology, but also many other sub-fields of physics, especially particle, solid-state and computational physics, as well as chemistry, geology and even biology. The present Astronuclear Physics review focusses primarily on the facets of nuclear physics that are of relevance to astronomy and astrophysics, the theoretical aspects being of special concern here.
The observational aspects of astronomy and astrophysics that may have some connection to nuclear physics are only broadly reviewed, mainly through the provision of recent relevant references. Multi-messenger astronomy has developed most remarkably during the last decades, with often direct implications for nuclear astrophysics. The electromagnetic view of the components of the Universe has improved dramatically at all wavelengths, from the γ-ray to the radio domains, providing important new information on the Big Bang and the properties of stars. Neutrino astronomy has made giant steps forward. In particular, the famed “solar neutrino problem” is now behind us. The long-sought gravitational waves have at last been detected, with direct relevance namely to the merger of compact stars. The composition of Galactic Cosmic Rays and stellar/solar energetic particles is better known than ever, providing constrains on the GCR physics.
On the stellar modeling side, we broadly brush the progress that has been made based on new observations, and even more so on the spectacular increase in computer capabilities. We briefly outline recent advances regarding the quiescent evolution of stars, as well as the eventual catastrophic supernova explosion of certain classes of them. In spite of significant improvements in the simulations, many long-standing problems still await solid solutions, particularly regarding the details and robustness of explosion simulations. In fact, new questions are continuously emerging, and new facts may endanger old ideas.
The lion’s share of this review concerns the nuclear physics phenomena that may be at work in astrophysical conditions, with a strong focus on theory. Exceptionally large varieties of nuclei have to be dealt with, ranging from the lightest to the heaviest ones, from the valley of nuclear stability all the way to the proton and neutron drip lines. An additional serious difficulty comes from the fact that the nuclei are immersed in highly unusual environments which may have a significant impact on their static properties, the diversity of their transmutation modes, some of which not being observable in the laboratory, and on the probabilities of these modes. The description of nuclei as individual entities has even to be replaced by the construction of an Equation of State at high enough temperatures and/or densities prevailing in the cores of exploding stars and in compact objects (neutron stars). The determination of a huge body of thermonuclear reaction cross sections is an especially challenging task, having to face the “world of almost no event” due to the smallness of the relative energies of charged-particle induced reactions relative to the Coulomb barriers, and/or the “world of exoticism”, as highly unstable nuclei are involved in several nucleosynthesis processes.
The synthesis of the nuclides heavier than iron is briefly reviewed. Neutron capture mechanisms range from the s-process for the production of the stable nuclides located at the bottom of the valley of stability to the r-process responsible for the synthesis of the neutron-rich isobars. The origin of the neutron-deficient isobars observed in the SoS is attributed to the p-process. Emphasis is put on the astrophysics and nuclear physics uncertainties affecting the modeling of these nucleosynthesis mechanisms.
We present the first comprehensive study of r-process element nucleosynthesis in the ejecta of compact binary mergers (CBMs) and their relic black hole (BH)–torus systems. The evolution of the ...BH–accretion tori is simulated for seconds with a Newtonian hydrodynamics code including viscosity effects, pseudo-Newtonian gravity for rotating BHs, and an energy-dependent two-moment closure scheme for the transport of electron neutrinos and antineutrinos. The investigated cases are guided by relativistic double neutron star (NS–NS) and NS–BH merger models, producing ∼3–6 M⊙ BHs with rotation parameters of A
BH ∼ 0.8 and tori of 0.03–0.3 M⊙. Our nucleosynthesis analysis includes the dynamical (prompt) ejecta expelled during the CBM phase and the neutrino and viscously driven outflows of the relic BH–torus systems. While typically ∼20–25 per cent of the initial accretion-torus mass are lost by viscously driven outflows, neutrino-powered winds contribute at most another ∼1 per cent, but neutrino heating enhances the viscous ejecta significantly. Since BH–torus ejecta possess a wide distribution of electron fractions (0.1–0.6) and entropies, they produce heavy elements from A ∼ 80 up to the actinides, with relative contributions of A ≳ 130 nuclei being subdominant and sensitively dependent on BH and torus masses and the exact treatment of shear viscosity. The combined ejecta of CBM and BH–torus phases can reproduce the solar abundances amazingly well for A ≳ 90. Varying contributions of the torus ejecta might account for observed variations of lighter elements with 40 ≤ Z ≤ 56 relative to heavier ones, and a considerable reduction of the prompt ejecta compared to the torus ejecta, e.g. in highly asymmetric NS–BH mergers, might explain the composition of heavy-element deficient stars.
We investigate systematically the dynamical mass ejection, r-process nucleosynthesis, and properties of electromagnetic counterparts of neutron-star (NS) mergers in dependence on the uncertain ...properties of the nuclear equation of state (EOS) by employing 40 representative, microphysical high-density EOSs in relativistic, hydrodynamical simulations. The crucial parameter determining the ejecta mass is the radius R sub(1.35) of a 1.35 M sub(middot in circle) NS. NSs with smaller R sub(1.35) ("soft" EOS) eject systematically higher masses. The r-process nucleosynthesis exhibits a remarkable robustness independent of the EOS, producing a nearly solar abundance pattern above mass number 130. By the r-process content of the Galaxy and the average production per event the Galactic merger rate is limited to 4 x 10 super(-5) yr super(-1) (4 x 10 super(-4) yr super(-1)) for a soft (stiff) NS EOS, if NS mergers are the main source of heavy super(r)-nuclei.
Context. An equation of state (EoS) of dense nuclear matter is a prerequisite for studies of the structure and evolution of compact stars. A unified EoS should describe the crust and the core of a ...neutron star using the same physical model. The Brussels-Montreal group has recently derived a family of such EoSs based on the nuclear energy-density functional theory with generalized Skyrme effective forces that have been fitted with great precision to essentially all the available mass data. At the same time, these forces were constrained to reproduce microscopic calculations of homogeneous neutron matter based on realistic two- and three-nucleon forces. Aims. We represent basic physical characteristics of the latest Brussels-Montreal EoS models by analytical expressions to facilitate their inclusion in astrophysical simulations. Methods. We consider three EoS models, which significantly differ by stiffness: BSk19, BSk20, and BSk21. For each of them we constructed two versions of the EoS parametrization. In the first version, pressure P and gravitational mass density ρ are given as functions of the baryon number density nb. In the second version, P, ρ, and nb are given as functions of pseudo-enthalpy, which is useful for two-dimensional calculations of stationary rotating configurations of neutron stars. In addition to the EoS, we derived analytical expressions for several related quantities that are required in neutron-star simulations: number fractions of electrons and muons in the stellar core, nucleon numbers per nucleus in the inner crust, and equivalent radii and shape parameters of the nuclei in the inner crust. Results. We obtain analytical representations for the basic characteristics of the models of cold dense matter, which are most important for studies of neutron stars. We demonstrate the usability of our results by applying them to calculations of neutron-star mass-radius relations, maximum and minimum masses, thresholds of direct Urca processes, and the electron conductivity in the neutron-star crust.
We investigate β-interactions of free nucleons and their impact on the electron fraction (Y
e) and r-process nucleosynthesis in ejecta characteristic of binary neutron star mergers (BNSMs). For that ...we employ trajectories from a relativistic BNSM model to represent the density–temperature evolutions in our parametric study. In the high-density environment, positron captures decrease the neutron richness at the high temperatures predicted by the hydrodynamic simulation. Circumventing the complexities of modelling three-dimensional neutrino transport, (anti)neutrino captures are parametrized in terms of prescribed neutrino luminosities and mean energies, guided by published results and assumed as constant in time. Depending sensitively on the adopted νe–
$\bar{\nu }_{\rm e}$
luminosity ratio, neutrino processes increase Y
e to values between 0.25 and 0.40, still allowing for a successful r-process compatible with the observed solar abundance distribution and a significant fraction of the ejecta consisting of r-process nuclei. If the νe luminosities and mean energies are relatively large compared to the
$\bar{\nu }_{\rm e}$
properties, the mean Y
e might reach values >0.40 so that neutrino captures seriously compromise the success of the r-process. In this case, the r-abundances remain compatible with the solar distribution, but the total amount of ejected r-material is reduced to a few per cent, because the production of iron-peak elements is favoured. Proper neutrino physics, in particular also neutrino absorption, have to be included in BNSM simulations before final conclusions can be drawn concerning r-processing in this environment and concerning observational consequences like kilonovae, whose peak brightness and colour temperature are sensitive to the composition-dependent opacity of the ejecta.
The pivotal role of nuclear physics in nucleosynthesis processes is being investigated, in particular the intricate influence of photon strength functions (PSFs) and nuclear level densities (NLDs) on ...shaping the outcomes of the i-, r- and p-processes. Exploring diverse NLD and PSF model combinations uncovers large uncertainties for (p, γ ), (n, γ ) and ( α , γ ) rates across many regions of the nuclear chart. These lead to potentially significant abundance variations of the nucleosynthesis processes and highlight the importance of accurate experimental nuclear data. Theoretical insights and advanced experimental techniques lay the ground work for profound understanding that can be gained of nucleosynthesis mechanisms and the origin of the elements. Recent results further underscore the effect of PSF and NLD data and its contribution to understanding abundance distributions and refining knowledge of the intricate nucleosynthesis processes. This article is part of the theme issue ‘The liminal position of Nuclear Physics: from hadrons to neutron stars’.
ABSTRACT
Black hole (BH) accretion discs formed in compact-object mergers or collapsars may be major sites of the rapid-neutron-capture (r-)process, but the conditions determining the electron ...fraction (Ye) remain uncertain given the complexity of neutrino transfer and angular-momentum transport. After discussing relevant weak-interaction regimes, we study the role of neutrino absorption for shaping Ye using an extensive set of simulations performed with two-moment neutrino transport and again without neutrino absorption. We vary the torus mass, BH mass and spin, and examine the impact of rest-mass and weak-magnetism corrections in the neutrino rates. We also test the dependence on the angular-momentum transport treatment by comparing axisymmetric models using the standard α-viscosity with viscous models assuming constant viscous length-scales (lt) and 3D magnetohydrodynamic (MHD) simulations. Finally, we discuss the nucleosynthesis yields and basic kilonova properties. We find that absorption pushes Ye towards ∼0.5 outside the torus, while inside increasing the equilibrium value $Y_\mathrm{ e}^{\mathrm{eq}}$ by ∼0.05–0.2. Correspondingly, a substantial ejecta fraction is pushed above Ye = 0.25, leading to a reduced lanthanide fraction and a brighter, earlier, and bluer kilonova than without absorption. More compact tori with higher neutrino optical depth, τ, tend to have lower $Y_\mathrm{ e}^{\mathrm{eq}}$ up to τ ∼ 1–10, above which absorption becomes strong enough to reverse this trend. Disc ejecta are less (more) neutron rich when employing an lt = const. viscosity (MHD treatment). The solar-like abundance pattern found for our MHD model marginally supports collapsar discs as major r-process sites, although a strong r-process may be limited to phases of high mass-infall rates, $\dot{M}\, \, \raise0.14em\rm{\gt }\lower0.28em\rm{\sim }\, \, 2\times 10^{-2}$ M⊙ s−1.
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