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 circle dot, which exceeds the number of heavy elements ejected as jets in the neutron star merger scenario.

The pattern of nucleosynthesis during the explosion of a low-mass neutron star formed in a close binary system in the stripping scenario is considered. In the scenario considered the shock arising during the explosion is shown to strongly heat the expanding neutron star matter. The heavy nuclei produced at the preceding stage of nucleosynthesis are partially destroyed as a result of a sharp increase in the role of photonuclear reactions. It is shown that even short-term heating of the matter by the shock can exert a noticeable influence on the results of the synthesis of elements in the r-process in the inner crust matter, while explosive nucleosynthesis gives rise to new elements in the outer crust matter with mass numbers A from 50 to 130 .

We investigate the impact of forthcoming nuclear data on the predictions of the neutron star (NS) stripping model for short gamma-ray bursts. The main area to which we pay attention is the NS crust. We show that the uncertain properties of the NS equation of state can significantly influence the stripping time tstr, the main dynamical parameter of the model. Based on the known time delay (tstr≈1.7 s) between the peak of the gravitational wave signal GW170817 and the detection of gamma photons from GRB170817A, we obtain new restrictions on the nuclear matter parameters, in particular, the symmetry energy slope parameter: L<114.5MeV. In addition, we study the process of nucleosynthesis in the outer and inner crusts of a low-mass NS. We show that the nucleosynthesis is strongly influenced by both the forthcoming nuclear data and the equation of state of the NS matter.

The nucleosynthesis of heavy elements is calculated for two scenarios of neutron-star merger. Various global beta-decay models, including those based on the random-phase approximation (QRPA), relativistic quasiparticle RPA ( pn -RQRPA), and the finite-amplitude method (FAM), were employed in these calculations. It is shown that the application of various global models in calculations of nucleosynthesis leads to the formation of a realistic structure of the curve of abundances of chemical elements. In contrast to nucleosynthesis in the scenario of merger of equal-mass neutron stars, the formation of elements in matter of the outer crust upon the explosion of a low-mass neutron star is weakly model-dependent in the region from the first to the second peak. However, the abundance of elements depends greatly on the beta-decay model in a strong r-process. No systematic effect of the beta-decay model on the results of nucleosynthesis is revealed.

The explosion of a minimal-mass neutron star formed in the course of the evolution of a close binary of neutron stars markedly differing in mass is considered. The abundance of heavy elements produced during the expansion of matter of the external neutron-star core after the explosion of a low-mass neutron star is calculated. It is shown that, in this scenario, a weak r-process proceeds in the external core, leading to the formation of a ''light'' fraction of heavy elements.

In this paper, the role of the mass distribution of fission fragments in the formation of heavy elements is considered. Two models of the mass distribution of fission fragments have been analyzed in detail: the Kodama–Takahashi model with a predominantly asymmetric distribution, and a model based on nuclear systematics with an almost symmetric distribution of fission remnants, which considers fission neutrons. It is shown that, in the merger scenario of a neutron star, the agreement between the second peak on the calculated abundance curve and observations can be obtained only if the distribution of fission remnants is symmetrical and fission neutrons are accounted for.

Рассмотрен сценарий взрыва нейтронной звезды минимальной массы, образующейся в процессе эволюции тесной двойной системы нейтронных звезд, сильно различающихся по массе. Рассчитана распространенность тяжелых элементов, образованных в процессе разлета вещества внешней коры взорвавшейся маломассивной нейтронной звезды. Показано, что в этом сценарии во внешней коре протекает слабый r-процесс и образуется ''легкая'' фракция тяжелых элементов. \(\phantom{A}\)

The results of calculations of nucleosynthesis of heavy elements during the explosion of a low-mass neutron star are presented. The low-mass neutron star is formed as a result of the exchange of matter in the last stages of the evolution of a close binary system of neutron stars with a large initial mass asymmetry. Two variants of the scenario, which use different approximations of the equation of state of neutron star matter: BSk22 and BSk25, are considered. Their usage lead to different expansion dynamics of the shells of a low-mass neutron star. It is shown that the character of shock wave propagation and the abundance of heavy elements formed in the inner layers of the outer crust with 0.29<Y_e<0.45 for these two scenarios are different, although the composition of the outer crust before the explosion differs insignificantly. These differences in the scenarios result in a noticeable difference in the calculated abundances of heavy elements both in these layers and in the entire examined part of the outer crust.

In this work, nucleosynthesis of heavy nuclei in the r-process is modelled. The influence of beta-decay rates calculated within different theoretical models on abundance of heavy nuclei is studied. It is shown that dynamics of nucleosynthesis of new elements and final abundance of heavy nuclei strongly depend on the beta-decay model used in calculations of the r-process. A possibility of using astrophysical calculations for testing predicted nuclear data defined only theoretically is discussed.

Nucleosynthesis at the deflagration stage of a white dwarf is considered. Burning calculations have been made with the previously developed hydrodynamic model implemented in the FRONT3D code including turbulence. The trajectories of passive particles used thereafter for nucleosynthesis simulations have been created through the hydrodynamic calculations. The abundances of elements from oxygen to iron and nickel calculated in the developed nucleosynthesis model are shown to be in agreement with both observations and calculations based on other models.

We explore the appearance of light clusters at high densities of collapsing stellar cores. Special attention is paid to the unstable isotope H-4, which was not included in previous studies. The importance of light clusters in the calculation of rates for neutrino matter interaction is discussed. The main conclusion is that thermodynamic quantities are only weakly sensitive to the chemical composition. The change in pressure and hence the direct change in collapse dynamics will be minor. But the change in neutrino heating and neutronization processes can be significant.

The role of weakly bound neutral clusters, such as dineutrons and tetraneutrons, in matter of high density and high temperature is discussed. Under such conditions, which are characteristic of core-collapse supernovae, the lifetime of multineutrons may prove to be sufficiently long for them to have a pronounced effect on the formation of the chemical composition. The influence of the multineutron binding energy and other nuclear properties on the magnitude of the effect being considered is examined.

The dependence of the region of physically admissible values of uranium-thorium isotope ratios on the short-term change in nucleosynthesis rate, which increases just before the formation of the Solar system, is studied. An additional admissible region associated with the presence of the isotope 244Pu and the effect of this region on the calculation of the age of the Universe, TU, are considered within galactic-nucleosynthsis theory. The size of the admissible region and its dependence on the short-term increase in the rate of heavy-element production (enhancement of nucleosynthsis) before the formation of the Solar System is discussed along with the consistency of the predictions for the above ratios with the region of admissible values. It is shown that an enhancement of nucleosynthesis is necessary for attaining agreement between the calculated ratios of cosmochronometer nuclei and the region of admissible values, but this enhancement should not lead to an increase in the abundances of heavy elements that is greater than 1 to 3% of the total amount of synthesized heavy elements.

It is shown that superheavy elements may also be formed in the main r process responsible for the formation of the heaviest elements observed in nature. Under conditions of a high neutron density, the nucleosynthesis region lies close to the neutron drip line, so that the r process may circumvent the region where nuclei undergo spontaneous fissions and therefore have short lifetimes. However, a high induced-fission rate, which increases with the charge number, may prevent the nucleosynthesis wave from overcoming the region of isotopes heavier than curium, and the beta-decay chain leading to an increase in the charge number of product elements inevitably results in the spontaneous fission of the majority of product nuclei. Calculations of nucleosynthesis that were performed with available nuclear data within the scenario of a neutron-star merger reveal that only Z < 106 superheavy elements are formed. Their abundance at the end of the r process is commensurate with the abundance of uranium, but their lifetime does not exceed several years, so that they fast undergo decay.