arXiv subjects

A. V. Yudin

Publications and source records attributed to A. V. Yudin.

At least 19 recordsLinked to original sources

arXiv2026

Numerical Study of Polytropes with n=1 and Differential Rotation

The solution space of differentially rotating polytropes with n=1 has been studied numerically. The existence of three different types of configurations: from spheroids to thick tori, hockey puck-like bodies and spheroids surrounded by a torus, separate from or merging with the central body has been proved. It has been shown that the last two types appear only at moderate degrees of rotation differentiality, sigma~2. Rigid-body or weakly differential rotation, as well as strongly differential, have not led to any "exotic" types of configurations. Many calculated configurations have had extremely large values of parameter tau, which has raised the question of their stability with respect to fragmentation.

gr-qc↗

arXiv2025

Non-conservation of linear momentum in widely used hierarchical methods in gravitational gas dynamics

The paper considers the implementation of the fast multipole method (FMM) in the PHANTOM code for the calculation of forces in a self-gravitating system. The gravitational interaction forces are divided into short-range and long-range interactions depending on the value of the tree opening parameter of the hierarchical kd-tree. It is demonstrated that Newton's third law holds for any pair of cells of the kd-tree engaged in mutual interaction. However, for the entire system a linear momentum is not conserved. As a result, there is an unphysical force that causes the center of mass to migrate. For example, for a pair of neutron stars, the displacement of the system's center of mass is found to be comparable to the radii of the objects at times of a few tens of Keplerian revolutions. This displacement cannot be reduced by increasing the number of particles for values of the tree opening parameter greater than 0.2. For smaller values, the time required for the calculation is significantly longer.

astro-ph.SR↗

arXiv2024

Low-mass neutron star nucleosynthesis -- stripping scenario

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 (shock-wave nucleosynthesis). The number of heavy elements produced in a low-mass neutron star crust during the explosion is approximately equals 0.041 solar masses, which exceeds the number of heavy elements ejected as jets in the neutron star merger scenario.

astro-ph.HE↗

arXiv2023

Neutrinos from pre-supernova in the framework of TQRPA method

We propose a new method for calculating spectra and luminosities for (anti)neutrinos produced in the pre-supernova environment by weak processes with hot nuclei. It is based on the thermal quasiparticle random phase approximation (TQRPA), that allows microscopic thermodynamically consistent calculations of the weak-interaction response of nuclei at finite temperatures. For realistic representative pre-supernova conditions from the stellar evolution code MESA, we compute (anti)neutrino luminosities and spectra arising from neutral- and charged-current weak reactions with hot $^{56}$Fe and compare them with the contribution of thermal processes. We find that the TQRPA approach produces not only a higher total luminosity of electron neutrinos (mainly born in the electron capture reaction), compared to the standard technique based on the large-scale shell model (LSSM) weak-interaction rates, but also a harder neutrino spectrum. Besides, applying the TQRPA and LSSM, we find that in the context of electron antineutrino generation, the neutral-current nuclear de-excitation (ND) process via neutrino-antineutrino pair emission is at least as important as the electron-positron pair annihilation process. We also show that flavor oscillations enhance the high-energy contribution of the ND process to the electron antineutrino flux. This could potentially be important for pre-supernova antineutrino registration by the Earth's detectors.

nucl-th↗

arXiv2023

Neutrino spectrum and energy loss rates due to weak processes on hot $^{56}$Fe in pre-supernova environment

Applying TQRPA calculations of Gamow--Teller strength functions in hot nuclei, we compute the (anti)neutrino spectra and energy loss rates arising from weak processes on hot $^{56}$Fe under pre-supernova conditions. We use a realistic pre-supernova model calculated by the stellar evolution code MESA. Taking into account both charged and neutral current processes, we demonstrate that weak reactions with hot nuclei can produce high-energy (anti)neutrinos. We also show that, for hot nuclei, the energy loss via (anti)neutrino emission is significantly larger than that for nuclei in their ground state. It is found that the neutral current de-excitation via the $ν\barν$-pair emission is presumably a dominant source of antineutrinos. In accordance with other studies, we confirm that the so-called single-state approximation for neutrino spectra might fail under certain pre-supernova conditions. }

nucl-th↗

arXiv2023

Thermal Neutrinos from the Explosion of a Minimum-Mass Neutron Star

We present our calculations of the thermal neutrino radiation that accompanies the explosion of a minimum-mass neutron star. In this case, the neutrino luminosity is lower than the luminosity during a supernova explosion approximately by five orders of magnitude, while the energy carried away by neutrinos is low compared to the explosion energy. We also show that the energy losses through neutrinos do not hinder the envelope heating and the cumulation of the shock during its breakout and the acceleration of the outer part of the envelope to ultrarelativistic speeds.

astro-ph.HE↗

arXiv2022

Explosion of a Minimum-Mass Neutron Star within Relativistic Hydrodynamics

The relativistic hydrodynamics equations are adapted for the spherically symmetric case and the Lagrangian form. They are used to model the explosive disruption of a minimum-mass neutron star: a key ingredient of the stripping model for short gamma-ray bursts. The shock breakout from the neutron star surface accompanied by the acceleration of matter to ultrarelativistic velocities is studied. A comparison with the results of previously published nonrelativistic calculations is made.

astro-ph.HE↗

arXiv2022

Simulation of the LSD Response to the Neutrino Burst from SN 1987A

Using the Geant4 code, we have performed a full-scale simulation of the LSD response to the neutrino burst from SN1987A. The neutrino flux parameters were chosen according to one of the models: the standard collapse model or the rotational supernova explosion model. We showed that, depending on the chosen parameters, one can either obtain the required number of pulses in the detector or reproduce their energy spectrum, but not both together. The interaction of neutrino radiation both with LSD itself and with the material of the surrounding soil was taken into account in our simulation. We also explored the hypothesis that the entire unique LSD signal at 2:52 UT was produced by neutron fluxes from the surrounding granite. However, this hypothesis was not confirmed by our simulation. The results obtained provide a rich material for possible interpretations.

astro-ph.HE↗

arXiv2021

Structure of Relativistic Stars Composed of Incompressible Matter in the Absence of Strict Electroneutrality

The structure of a star composed of locally non-electroneutral incompressible three-component matter is considered within the framework of general relativity. For thermodynamic quantities like the pressure, the solution can be represented as a series in the small parameter $1/Λ_{\mathrm G}\sim 10^{-36}$, where the first approximation is the well-known electroneutral solution. However, the equilibrium equations for the chemical potentials of the matter components, as it turns out, contain finite contributions from non-electroneutrality effects even in the zeroth order. Analytical solutions have been obtained for all of the parameters of the problem under consideration, which are illustrated by numerical examples.

astro-ph.HE↗

arXiv2020

Production of Heavy Elements During the Explosion of a Low-Mass Neutron Star in a Close Binary

The nucleosynthesis of heavy elements in the scenario for the evolution of a close binary of neutron stars differing greatly in mass is considered. In contrast to the scenario for the merger of two neutron stars of comparable masses considered repeatedly in the literature, the evolution of such a binary at the final stage consists in a rapid mass transfer to the more massive star and an explosive disruption of the low-mass component. We provide the details of the explosion and calculate the abundances of the heavy elements produced in this process for various initial conditions.

nucl-th↗

arXiv2020

Low-Mass Neutron Stars with Rotation

The properties of low-mass neutron stars with rigid rotation are considered. The possible evolution paths of such stars in a close binary system with mass transfer are calculated. The properties of the gamma-ray burst GRB 170817A interpreted in terms of the stripping model, a natural ingredient of which is the explosion of a low-mass neutron star - a binary component, are briefly discussed.

astro-ph.HE↗

arXiv2020

Light Neutral Clusters in Supernova Matter

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.

nucl-th↗

arXiv2019

A simple model of time-dependent ionization in Type IIP supernova envelopes

We propose a model kinetic system of the hydrogen atom (two levels plus continuum) under the conditions typical for atmospheres of Type IIP supernovae in the plateau stage. Despite the simplicity of this system, it describes realistically the basic properties of the complete system. Analysis shows that the ionization "freeze-out" effect is always manifest at large times. We give a simple criterion for checking the statistical equilibrium of a system under the given conditions at any time. It is shown that if the system is in non-equilibrium at early times, the time-dependent effect of ionization necessarily exists. We also generalize this criterion to the case of arbitrary multilevel systems. We discuss various factors that affect the strength of the time-dependent effect in the kinetics during the photospheric phase of a supernova explosion.

astro-ph.SR↗

arXiv2019

Dynamics of supernova bounce in laboratory

We draw attention to recent high explosive (HE) experiments which provide compression of macroscopic amount of matter to high, even record, values of pressure in comparison with other HE experiments. The observed bounce after the compression corresponds to processes in core-collapse supernova explosions after neutrino trapping. Conditions provided in the experiments resemble those in core-collapse supernovae, permitting their use for laboratory astrophysics. A unique feature of the experiments is compression at low entropy. The values of specific entropy are close to those obtained in numerical simulations during the process of collapse in supernova explosions, and much lower than those obtained at laser ignition facilities, another type of high-compression experiment. Both in supernovae and HE experiments the bounce occurs at low entropy, so the HE experiments provide a new platform to realize some supernova collapse effects in laboratory, especially to study hydrodynamics of collapsing flows and the bounce. Due to the good resolution of diagnostics in the compression of macroscopic amounts of material with essential effects of nonideal plasma in EOS, and observed development of 3D instabilities, these experiments may serve as a useful benchmark for astrophysical hydrodynamic codes.

physics.plasm-ph↗

arXiv2018

Hydrostatic equilibrium of stars without electroneutrality constraint

The general solution of hydrostatic equilibrium equations for a two-component fluid of ions and electrons without a local electroneutrality constraint is found in the framework of Newtonian gravity theory. In agreement with the Poincaré theorem on analyticity and in the context of Dyson's argument, the general solution is demonstrated to possess a fixed (essential) singularity in the gravitational constant $G$ at $ G = 0 $. The regular component of the general solution can be determined by perturbation theory in $G$ starting from a locally neutral solution. The non-perturbative component obtained using the method of Wentzel, Kramers and Brillouin is exponentially small in the inner layers of the star and grows rapidly in the outward direction. Near the surface of the star, both components are comparable in magnitude, and their non-linear interplay determines the properties of an electro- or ionosphere. The stellar charge varies within the limits of $- 0.1 $ to $150$ C per solar mass. The properties of electro- and ionospheres are exponentially sensitive to variations of the fluid densities in the central regions of the star. The general solutions of two exactly solvable stellar models without a local electroneutrality constraint are also presented.

astro-ph.SR↗

arXiv2017

Neutron Production by Cosmic-Ray Muons in Various Materials

The results obtained by studying the background of neutrons produced by cosmic-ray muons in underground experimental facilities intended for rare-event searches and in surrounding rock are presented. The types of this rock may include granite, sedimentary rock, gypsum, and rock salt. Neutron production and transfer were simulated using the Geant4 and SHIELD transport codes. These codes were tuned via a comparison of the results of calculations with experimental data: in particular, with data of the Artemovsk research station of the Institute for Nuclear Research (INR, Moscow, Russia), as well as via an intercomparison of results of calculations with the Geant4 and SHIELD codes. It turns out that the atomic-number dependence of the production and yield of neutrons has an irregular character and does not allow a description in terms of a universal function of the atomic number. The parameters of this dependence are different for two groups of nuclei-nuclei consisting of alpha particles and all of the remaining nuclei. Moreover, there are manifest exceptions from a power-law dependence, for example, argon. This may entail important consequences both for the existing underground experimental facilities and for those under construction. Investigation of cosmic-ray-induced neutron production in various materials is of paramount importance for the interpretation of experiments conducted at large depths under the Earth's surface.

physics.ins-det↗

arXiv2017

Variational Principle for Stars with a Phase Transition

The variational principle for stars with a phase transition has been investigated. The term outside the integral in the expression for the second variation of the total energy of a star is shown to be obtained by passage to the limit from the integration over the region of mixed states in the star. The form of the trial functions ensuring this passage has been found. All of the results have been generalized to the case where general relativity is applicable. The known criteria for the dynamical stability of a star when a new phase appears at its center are shown to follow automatically from the variational principle. Numerical calculations of hydrostatically equilibrium models for hybrid stars with a phase transition have been performed. The form of the trial functions for the second variation of the total energy of a star that describes almost exactly the stability boundaries of such stellar models is proposed.

astro-ph.SR↗

arXiv2017

Neutrino Fluxes from a Core-Collapse Supernova in a Model with Three Sterile Neutrinos

The characteristics of the gravitational collapse of a supernova and the fluxes of active and sterile neutrinos produced during the formation of its protoneutron core have been calculated numerically. The relative yields of active and sterile neutrinos in core matter with different degrees of neutronization have been calculated for various input parameters and various initial conditions. A significant increase in the fraction of sterile neutrinos produced in superdense core matter at the resonant degree of neutronization has been confirmed. The contributions of sterile neutrinos to the collapse dynamics and the total flux of neutrinos produced during collapse have been shown to be relatively small. The total luminosity of sterile neutrinos is considerably lower than the luminosity of electron neutrinos, but their spectrum is considerably harder at high energies.

astro-ph.HE↗