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The macroscopic model for a neutron star (NS) as a liquid drop at the equilibrium is used to extend the Tolman-Oppenheimer-Volkoff (TOV) equations taking into account the gradient terms responsible for the system surface. The parameters of the Schwarzschild metric in the spherical case are found with these surface corrections in the leading (zero) order of the leptodermic approximation $a/R<<1$, where $a$ is the NS effective-surface (ES) thickness, and $R$ is the effective NS radius. The energy density $\mathcal{E}$ is considered in a general form including the functions of the particle number density and of its gradient terms. The macroscopic gravitational potential $\Phi(\rho)$ is taken into account in the simplest form as expansion in powers of $\rho-\overline{\rho} $, where $\overline{\rho}$ is the saturation density, up to second order, in terms of its contributions to th separation particle energy and incompressibility. Density distributions $\rho$ across the NS ES in the normal direction to the ES, which are derived in the simple analytical form at the same leading approximation, was used for the derivation of the modified TOV (MTOV) equations by accounting for their NS surface corrections. The MTOV equations are analytically solved at first order and the results are compared with the standard TOV approach of the zero order., Comment: 24 pages, 9 figures, 1 table

The macroscopic model for a neutron star (NS) as a perfect liquid drop at the equilibrium is formulated for derivations of the particle number density $\rho$, equation of state (EoS) for the pressure $P=P(\rho)$, and the NS mass expressions, $M=M(R)$. We use the leptodermic approximation $a/R<<1$, where $a$ is the crust thickness of the effective NS surface (ES), and $R$ is the mean radius of the ES curvature. Within the approximate Schwarzschild metric solution to the general relativity theory equations for the spherically symmetric systems, the macroscopic gravitation is taken into account in terms of the total separation energy and incompressibility in the simplest form as expansion of a gravitational potential $\Phi(\rho)$ in powers of $\rho-\overline{\rho} $, where $\overline{\rho}$ is the saturation density, up to second order. Density distribution $\rho$ across the ES in the normal direction to the ES was obtained analytically for a general form of the energy density $\mathcal{E}(\rho)$ in the leading order of parameter $a/R \ll 1$. For the typical crust thickness, $a \sim 1$ km, and effective radii, $R\sim 10$ km, one finds the leading expression for the density $\rho$. NS masses are analytically calculated as a sum of the volume and surface terms, $M(R)=M_{\rm V}+M_{\rm S}$, taking into account the radial curvature of the Schwarzschild metric space, in reasonable agreement with recently measured masses, $M=(1.4-2.0) M_\odot$, and radii, $R=10-14$ km, for several neutron stars. The energy tension coefficient is derived with accounting for the Schwarzschild metric correction. We derive the simple macroscopic equation of state, $P=P(\rho)$, with the surface correction. The analytical and numerical solutions to Tolman-Oppenheimer-Volkoff equations for the pressure are in good agreement with the volume part of our EoS., Comment: 34 pages, 10 figures

Level density $\rho$ is derived for a finite system with strongly interacting nucleons at a given energy E, neutron N and proton Z particle numbers, projection of the angular momentum M, and other integrals of motion, within the semiclassical periodic-orbit theory (POT) beyond the standard Fermi-gas saddle-point method. For large particle numbers, one obtains an analytical expression for the level density which is extended to low excitation energies U in the statistical micro-macroscopic approach (MMA).The interparticle interaction averaged over particle numbers is taken into account in terms of the extended Thomas-Fermi component of the POT. The shell structure of spherical and deformed nuclei is taken into account in the level density. The MMA expressions for the level density $\rho$ reaches the well-known macroscopic Fermi-gas asymptote for large excitation energies U and the finite combinatoric power-expansion limit for low energies U. We compare our MMA results for the averaged level density with the experimental data obtained from the known excitation energy spectra by using the sample method under statistical and plateau conditions. Fitting the MMA $\rho$ to these experimental data on the averaged level density by using only one free physical parameter - inverse level density parameter K - for several nuclei and their long isotope chain at low excitation energies U, one obtains the results for K. These values of K might be much larger than those deduced from neutron resonances. The shell, isotopic asymmetry, and pairing effects are significant for low excitation energies., Comment: 31 pages, 7 figures, 1 table

Level density $\rho(E,N,Z)$ is calculated for the two-component close- and open-shell nuclei with a given energy $E$, and neutron $N$ and proton $Z$ numbers, taking into account pairing effects within the microscopic-macroscopic approach (MMA). These analytical calculations have been carried out by using the semiclassical statistical mean-field approximations beyond the saddle-point method of the Fermi gas model in a low excitation-energies range. The level density $\rho$, obtained as function of the system entropy $S$, depends essentially on the condensation energy $E_{\rm cond}$ through the excitation energy $U$ in super-fluid nuclei. The simplest super-fluid approach, based on the BCS theory, accounts for a smooth temperature dependence of the pairing gap $\Delta$ due to particle number fluctuations. Taking into account the pairing effects in magic or semi-magic nuclei, excited below neutron resonances, one finds a notable pairing phase transition.Pairing correlations sometimes improve significantly the comparison with experimental data., Comment: 8 pages, 2 figures, 2 tables

The equation of state with quantum statistics corrections is used for particle number fluctuations $\omega$ of isotopically symmetric nuclear matter with interparticle van der Waals and Skyrme local density interactions. The fluctuations, $\omega\propto 1/\mathcal{K}$, are analytically derived through the isothermal incompressibility $\mathcal{K}$ at first order over a small quantum-statistics parameter. Our approximate analytical results appear to be in good agreement with the results of accurate numerical calculations. These results are also close to those obtained by using more accurate Tolman and Rowlinson expansions of the incompressibility $\mathcal{K}$ near the critical point. A more general formula for fluctuations $\omega$, improved at the critical point, was obtained for a finite particle-number average $\langle N \rangle$ by neglecting, for simplicity, small quantum statistics effects. It is shown that for a large dimensionless parameter, $\alpha \propto \mathcal{K}^2\langle N \rangle/\mathcal{K}^{\prime\prime} $, where $\mathcal{K}^{\prime\prime}$ is the second derivative of the incompressibility $\mathcal{K}$ as function of the average particle density $n$, far from the critical point ($\alpha \gg 1$), one finds the traditional asymptote, $\omega\propto 1/\mathcal{K}$, for the fluctuations $\omega$. For a small parameter, $\alpha \ll 1$, near the critical point, where $\mathcal{K}=0$ and $\alpha=0$, one obtains another asymptote of $\omega$. These fluctuations, having a maximum near the critical point as function of the average density $n$, for finite values of $\langle N \rangle$ are finite and relatively small, in contrast to the results of the traditional calculations., Comment: 22 pages, 10 figures, 2 tables

Level density $\rho(E,{\bf Q})$ is derived within the micro-macroscopic approximation (MMA) for a system of strongly interacting Fermi particles with the energy $E$ and additional integrals of motion ${\bf Q}$, in line with several topics of the universal and fruitful activity of A.S. Davydov. Within the extended Thomas Fermi and semiclassical periodic orbit theory beyond the Fermi-gas saddle-point method we obtain $\rho\propto I_\nu(S)/S^\nu$, where $I_\nu(S)$ is the modified Bessel function of the entropy $S$. For small shell-structure contribution one finds $\nu=\kappa/2+1$, where $\kappa$ is the number of additional integrals of motion. This integer number is a dimension of ${\bf Q}$, ${\bf Q}=\{N, Z, ...\}$ for the case of two-component atomic nuclei, where $N$ and $Z$ are the numbers of neutron and protons, respectively. For much larger shell structure contributions, one obtains, $\nu=\kappa/2+2$. The MMA level density $\rho$ reaches the well-known Fermi gas asymptote for large excitation energies, and the finite micro-canonical combinatoric limit for low excitation energies. The additional integrals of motion can be also the projection of the angular momentum of a nuclear system for nuclear rotations of deformed nuclei, number of excitons for collective dynamics, and so on. Fitting the MMA total level density, $\rho(E,{\bf Q})$, for a set of the integrals of motion ${\bf Q}=\{N, Z\}$, to experimental data on a long nuclear isotope chain for low excitation energies, one obtains the results for the inverse level-density parameter $K$, which differs significantly from those of neutron resonances, due to shell, isotopic asymmetry, and pairing effects., Comment: 24 pages, 4 figures, 1 table. arXiv admin note: substantial text overlap with arXiv:2109.01830

Equation of state with quantum statistics corrections is derived for systems of the Fermi and Bose particles by using their van der Waals (vdW) and effective density-dependent Skyrme mean-field interactions. First few orders of these corrections over the small quantum statistics parameter, $\varepsilon \approx \hbar^3 n(mT)^{-3/2}g^{-1}$, where $n$ and $T$ are the particle number density and temperature, $m$ and $g$ the mass and degeneracy factor of particles, are analytically obtained. For interacting system of nucleon and $\alpha$ - particles, a small impurity of $\alpha$ - particles to a nucleon system at leading first order in both $\alpha $-particle and nucleon small parameters $\varepsilon$ does not change much the basic results for the symmetric nuclear matter in the quantum vdW consideration. Our approximate analytical results for the quantum vdW and Skyrme mean-field approaches are in a good agreement with accurate numerical calculations., Comment: 10 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:2012.09695

Equation of state with quantum statistics corrections is derived for a multi-component gas of particles interacting through the repulsive and attractive van der Waals (vdW) forces up to first few orders over a small parameter $\delta \approx \hbar^3 n(mT)^{-3/2}[g(1- bn)]^{-1}$, where $n$ and $T$ are the particle number density and temperature, $m$ and $g$ the particle mass and degeneracy factor. The parameter $b$ corresponds to the vdW excluded volume. For interacting system of Fermi nucleon and Bose $\alpha$ particles, a small impurity of $\alpha$ particles to the nucleon system at leading first order in both $\alpha $ particle and nucleon small parameters $\delta $ does not change much the basic results for the symmetric nuclear matter. The particle number fluctuations $\omega$ determined by the isothermal in-compressibility $\mathcal{K}(n,T)$ can be obtained analytically at the same first order quantum-statistics approximation for symmetric nucleon matter. Our approximate analytical results appear to be in good agreement with the accurate numerical calculations., Comment: 12 pages, 5 figures, 2 tables

Velocity and absorption coefficient of the plane sound waves in classical gases are obtained by solving the Boltzmann kinetic equation. This is done within the linear response theory as a reaction of the single-particle distribution function to a periodic external field. The nonperturbative dispersion equation is derived in the relaxation time approximation and solved numerically. The obtained theoretical results demonstrate an universal dependence of the sound velocity and scaled absorption coefficient on variable $\omega\tau$, where $\omega$ is the sound frequency and $\tau^{-1}$ is the particle collision frequency. In the region of $\omega\tau\sim 1$ a transition the frequent- to rare-collision regimes takes place. The sound velocity increases sharply, and the scaled absorption coefficient has a maximum -- both theoretical findings are in agreement with the data., Comment: 8 pages, 2 figures

The velocity and absorption coefficient for the plane sound waves in a classical gas are obtained by solving the Boltzmann kinetic equation, which describes the reaction of the single-particle distribution function to a periodic external field. Within the linear response theory, the nonperturbative dispersion equation valid for all sound frequencies is derived and solved numerically. The results are in agreement with the approximate analytical solutions found for both the frequent- and rare-collision regimes., Comment: 8 pages, 2 figures

The shear viscosity $\eta$ for a dilute classical gas of hard-sphere particles is calculated by solving the Boltzmann kinetic equation in terms of the weakly absorbed plane waves. For the rare-collision regime, the viscosity $\eta$ as a function of the equilibrium gas parameters -- temperature $T$, particle number density $n$, particle mass $m$, and hard-core particle diameter $d$ -- is quite different from that of the frequent-collision regime, e.g., from the well-known result of Chapman and Enskog. An important property of the rare-collision regime is the dependence of $\eta$ on the external ("non-equilibrium") parameter $\omega$, frequency of the sound plane wave, that is absent in the frequent-collision regime at leading order of the corresponding perturbation expansion. A transition from the frequent to the rare-collision regime takes place when the dimensionless parameter $nd^2 (T/m)^{1/2} \omega^{-1}$ goes to zero., Comment: 7 pages, 1 figure

Shear viscosity $\eta$ is calculated for the nuclear matter described as a system of interacting nucleons with the van der Waals (VDW) equation of state. The Boltzmann-Vlasov kinetic equation is solved in terms of the plane waves of the collective overdamped motion. In the frequent-collision regime, the shear viscosity depends on the particle-number density $n$ through the mean-field parameter $a$, which describes attractive forces in the VDW equation. In the temperature region $T=15 - 40$~MeV, a ratio of the shear viscosity to the entropy density $s$ is smaller than 1 at the nucleon number density $n =(0.5 - 1.5)\,n^{}_0$, where $n^{}_0=0.16\,$fm$^{-3}$ is the particle density of equilibrium nuclear matter at zero temperature. A minimum of the $\eta/s$ ratio takes place somewhere in a vicinity of the critical point of the VDW system. Large values of $\eta/s\gg 1$ are, however, found in both the low-density, $n\ll n^{}_0$, and high-density, $n>2n^{}_0$, regions. This makes the ideal hydrodynamic approach inapplicable for these densities., Comment: 12 pages, 5 figures