Dynamics of unstable sound waves in a non-equilibrium medium at the nonlinear stage

A new dispersion equation is obtained for a non-equilibrium medium with an exponential relaxation model of a vibrationally excited gas. We have researched the dependencies of the pump source and the heat removal on the medium thermodynamic parameters. The boundaries of sound waves stability regions in a non-equilibrium gas have been determined. The nonlinear stage of sound waves instability development in a vibrationally excited gas has been investigated within CSPH-TVD and MUSCL numerical schemes using parallel technologies OpenMP-CUDA. We have obtained a good agreement of numerical simulation results with the linear perturbations dynamics at the initial stage of the sound waves growth caused by instability. At the nonlinear stage, the sound waves amplitude reaches the maximum value that leads to the formation of shock waves system.Comment: 11 pages, 5 figure

Instability of high-frequency acoustic waves in accretion disks with turbulent viscosity

Dynamics of linear perturbations in a differentially rotating accretion disk with non-homogeneous vertical structure is investigated. It has been found that turbulent viscosity results in instability of both pinching oscillations, and bending modes. Not only the low-frequency fundamental modes, but also the high-frequency reflective harmonics appear to be unstable. The question of the limits of applicability of the thin disk model (MTD) is also investigated. The insignificant distinctions in the dispersion properties of MTD and three-dimensional model appear for wave numbers k<(1-3)/h (h is the half-thickness of a disk). In the long-wavelenght limit, the relative difference between eigenfrequencies of the unstable acoustic mode in the 3D-model and the MTD is smaller than 5%.Comment: Plain TeX, 9 pages, 3 figures (GIF), uses cp-aa.tex, Submitted to Astron. Astrophy

Self-consistent gas and stellar dynamics of disk galaxies: A problem of dark mass

We present results of numerical modeling made for the galactic stellar and stellar-gas disk embedded in the spherical halo and bulge. The stellar disk is simulated by N-body system, the equations of hydrodynamics are solved by TVD-method. We used TREEcode-algorithm for calculation of a self-gravity in stellar and gaseous components. The possibility of bars birth in a hot stellar disk because of gravitational instability of a cold gas component is investigated. The conditions of occurrence lopsided-galaxies from a axisymmetric disk as a result of gravitational instability are explored. The self-consistent models of double bars are constructed and the dynamical stability of these structures is discussed.Comment: 7 pages 6 figures, conference: 'Progress in Study of Astrophysical Disks: Collective and Stochastic Phenomena and Computational Tools

Disorder solutions for the free energy of the Ising-like models

For arbitrary Ising-like models of any dimension and Hamiltonians with a finite support with all possible multispin interactions and boundary conditions with a shift, the exact value of the free energy in the thermodynamic limit is obtained at some parametrically specified set of multispin interaction coefficients. In this case, half of the multispin interaction coefficients and the coordinates of the special eigenvector corresponding to the largest eigenvalue of the elementary transfer matrix are parameters, and the second half of the multispin coefficients is calculated using simple explicit formulas. For models with Hamiltonians invariant under the reversal of signs of all spins, the formulas are simplified. As examples of independent interest, solutions are written for the cases when the support of the Hamiltonian is a simplex, a cube, the support of the ANNNI model in spaces of 2, 3 and arbitrary dimensions.Comment: 35 pages, 3 figure

Interaction between collisionless galactic discs and nonaxissymmetric dark matter haloes

Using $N$-body simulations ($N\sim 10^6 - 10^7$), we examine how a non-axisymmetric dark halo affects the dynamical evolution of the structure in collisionless (stellar) discs. We demonstrate how the model parameters such as mass of the halo, initial conditions in the disc and the halo axes ratio affect morphology and kinematics of the stellar discs. We show that a non-axisymmetric halo can generate a large-scale spiral density pattern in the embedded stellar disc. The pattern is observed in the disc for many periods of its revolution, even if the disc is gravitationally over-stable. The growth of the spiral arms is not accompanied by significant dynamical heating of the disc, irrelevant to its initial parameters. We also investigate transformation of the dark halo's shape driven by the long-lived spiral pattern in the disc . We show that the analysis of the velocity field in the stellar disc and in the spiral pattern gives us a possibility to figure out the spatial orientation of the triaxial-shaped dark halo and to measure the triaxiality.Comment: 11 pages, 13 figures, accepted to MNRA

Analysis and prediction of changes in the temperature of the pure freshwater ice column in the Antarctic and the Arctic

This paper investigates the initial boundary value problem for a non-stationary one-dimensional heat equation that simulates the temperature distribution in freshwater ice near the Earth's poles. The mathematical model has been constructed taking into account solid-liquid phase transitions. Data from meteorological stations were used to determine the model parameters, with the help of which the necessary physical and thermophysical characteristics of the computational domain were obtained. For the numerical solution of the problem, the finite volume method (FVM) was used. For the obtained periodic regime, the temperature versus depth dependences for each month were plotted, and the depth of the active layer, as well as the depth of zero annual amplitudes were found for each meteorological station. A forecast of the ice temperature regime for 2100 was modeled for three Representative Concentration Pathway (RCP) scenarios of global warming: moderate RCP2.6, corresponding to the current emissions of RCP7 and adopted at the Paris Agreement in 2015 RCP1.9. By analyzing the impact of an additional 0,5 degrees Celsius of warming on other areas, a reduction in the full range of risks to humanity and the planet as a whole becomes evident with the proper efforts of the global community. Thus, the conducted modeling has confirmed the need to reduce the rate of global warming.Comment: 33 pages, 7 figure

Spiral structure of the Milky Way galaxy: observations and theoretical predictions

Using observational data on the kinematical properties and density distributions of the subsystems of the Milky Way galaxy, we construct a set of multi-component equilibrium models of its disk. The dynamics of the disk is studied numerically using collisionless-gaseous numerical simulations. After approximately one Gyr, a prominent central bar is formed with a semi-axis of about three kiloparsecs. Outside the central regions, a multi-armed spiral pattern develops, which can be characterized by the superposition of m=2, 3 and 4-armed spiral patterns. The spiral structure and the bar exist for at least 3 Gyr in our simulations. The presence of the bar in the disk of the MilkyWay galaxy imposes rather strict limitations on the density distributions in the subsystems of the MilkyWay galaxy.We find that bar does not form if the radial scale length of the surface density distribution of the disk is more than 2.6 kpc. Analogously, the formation of bar is suppressed in the Milky Way disk in models with a massive and compact stellar bulge. If future GAIA observations confirm the existence of the three-four armed spiral pattern in the disk of the MilkyWay, this will prove the long-term existence of spiral patterns in galactic disks.Comment: 6 pages, 3 figures, Conference Modern stellar astronomy-2019, SAO RAS, October 7-11, 2019 (Submitted: Research in Astronomy and Astrophysics

Formation of Ionization-Cone Structures in Active Galactic Nuclei: II. Nonlinear Hydrodynamic Modelling

In Part I of this paper we described an equilibrium model of a jet in the gravitational field corresponding to the rigid-rotation region of the galactic disk. We used linear stability analysis to find the waveguide-resonance instability of internal gravity waves due to the superreflection of these waves from the jet boundary. In this part of the paper, we perform nonlinear numerical 2D and 3D simulations of the development of this instability. We show that the shocks produced by this instability in the ambient medium of the jet are localized inside a cone with a large opening angle and are capable of producing features that are morphologically similar to those observed in galaxies with active nuclei (NGC 5252 for example).Comment: 10 pages, 9 PS and 4 JPEG figures, published in Astrophysical Bulleti

The structure and kinematics of AGN with ionization cones

Results of 2D spectral observations for 4 Sy galaxies with ionization cones are presented. Images in the [OIII] line, velocity fields of the ionized gas, stellar velocity and velocity dispersion fields were obtained at the SAO RAS 6m telescope. Non-circular gas motions and Z-shaped emission filaments could be explained as helical waves located in the ionization cone. This waves are generated by the hydrodynamical instability due to the velocity break between galactic ISM and outflowing matter from the AGN engine. The axis of cone lies close to the direction of a nuclear mini bar.Comment: 9 pages, 4 ps-figures (color), poster was presented at JENAM-2000 (Moscow); Corrected typo

Helical structures in Seyfert galaxies

The Seyfert galaxies with Z-shaped emission filaments in the Narrow Line Region (NLR) are considered. We assume that observable Z-shaped structures and velocity pattern of NLR may be explained as tridimensional helical waves in the ionization cone.Comment: To appear in IAU Colloq. 184, AGN Surveys, ed. R. F. Green, E. Ye. Khachikian, & D. B. Sanders (San Francisco: ASP), in press. 2 pages, 1 EPS figur