Isothermal Gas Clouds Research Articles

Influence of dark matter on gravitational stability of isothermal gas clouds

ABSTRACT To date, the presence of dark matter (DM) can be judged only by its gravitational interaction on the visible matter. It is therefore important to find the consequences of this interaction, which can then help to determine both the DM properties and parameters and the dynamics and evolution of visible matter. The gravitational influence of dark matter on the stability of interstellar medium (ISM), the progenitor of stars and star clusters, was considered. An isothermal self-gravity gas was taken as a suitable model describing ISM, particles interacting only gravitationally were considered as DM. The results obtained by analytical methods show that even a small amount of fast DM particles significantly increases the stable radius of the gas cloud and the corresponding mass while a higher relative density of DM destabilizes the gas. It was shown that with typical parameters of ISM and DM, its presence increases the maximum stable mass of isothermal cloud by a factor of 4 and the radius by five.

The cuFFT code for N-body simulation

The complexity of astrophysical processes lies in the joint consideration of components of various nature. For example, in the collision problem of galaxies, the three-dimensional dynamics of an interstellar gas and a stellar component is considered. The modeling of these components can be based on completely different classes of numerical methods. One possible solution to this problem is to use the Eulerian-Lagrangian approach, in which physical quantities are concentrated at material points, which is typical for the SPH (Smoothed Particle Hydrodynamics) method, and the forces are calculated on an adaptive grid attached to a system of material points. This approach uniformly takes into account both the dynamics of a continuous medium and discrete particles, and also eliminates a number of drawbacks inherent in the original method. The calculation of gravitational interaction is carried out by solving the Poisson equation for the gravitational potential. In this case, all particles are projected onto the computational grid and the potential values in each cell are already calculated on it. The solution of the Poisson equation for the gravitational potential is performed using the fast Fourier transform. The article describes the new cuFFT code Virtual Planetarium for modeling astrophysical objects based on the SPH method, supplemented by the Godunov method for calculating pressure and momentum flows between particles, and the fast Fourier transform method for solving the Poisson equation for the gravitational potential. The paper describes the rationale for the transition to such a computational model, kinetic and hydrodynamic approaches are described in detail. Simulation of the collapse of an isothermal gas cloud is performed. Method to reproduce the evolution of instabilities in form of two density arms is realized.

Код «Виртуальный планетарий» для моделирования астрофизических объектов: математическая модель, методология и первые результаты

The complexity of astrophysical processes lies in simultaneous consideration of components with different nature. So, for example, in the problem of collision of galaxies the three-dimensional dynamics of interstellar gas and stellar component is considered. The modeling of these components could be based on completely different classes of numerical methods. One possible solution to this problem is the use of the Eulerian-Lagrangian approach, in which the physical quantities are concentrated in material points, which is typical for the SPH method, and the calculation of forces is made on an adaptive grid bound to the system of material points. This approach uniformly takes into account both the dynamics of the continuous medium and discrete particles, and also eliminates a number of disadvantages typical for the original method. The calculation of the gravitational interaction is carried out by solving the Poisson equation for the gravitational potential. Herewith, all the particles are projected onto the computational grid and the potential values in each cell are calculated using this grid. The solution of the Poisson equation for the gravitational potential is performed using Fast Fourier Transform. The new “Virtual Planetarium” code for astrophysical objects modeling based on SPH method, supplemented by Godunov method for calculating pressure and momentum flows between particles, and Fast Fourier Transform method to solve the Poisson equation for the gravitational potential, is described in the paper. Rationale for the transition to such numerical model is given in the paper. Kinetic and hydrodynamic approaches are described in detail. The modeling of collapse of an isothermal gas cloud is performed, the ability of the method to reproduce the development of instabilities in the form of the formation of two density sleeves is shown.

The stability of the Circumnuclear Disk clouds in the Galactic Centre

The influence of rotation and magnetic fields on the physical properties of isothermal gas clouds is discussed. The presence of rotation and/or magnetic fields results in an increase of the critical cloud mass with respect to gravitational instability for clouds of a given temperature and external pressure. Rotating clouds have higher densities. Consequently, they are more stable against tidal shear than non-rotating clouds. They can approach the Galactic Centre up to a radius of ~2 pc without being disrupted by the tidal shear due to the gravitational potential. For smaller radii the clouds either collapse or become tidally disrupted. We suggest that this mechanism is responsible for the formation of the inner edge of the Circumnuclear Disk in the Galactic Centre.

Concentration measurements in dense isothermal gas clouds with different starting conditions

Experimental results are presented for instantaneous finite-volume dense-gas releases in a channel of 20 m length. The gases used were argon and argon-air mixtures with density ratios relative to air varying from 1.15 to 1.35. The aspect ratio (height to length) of the initial volume was varied from 0.42 to 1.68 with gas volumes from 1.89 to 3.78 m 3. Measurements with 50 fast-response gas sensors (aspirated hot-wires) gave concentration signals with good resolution in time and space. The data obtained allow the air entrainment rates (both local and global) to be determined and give detailed information on the influence of the initial aspect ratio. Comparisons with theoretical solutions from the shallow-layer approximation (code SHALADIS) and with a dynamic integral model (adapted from Van Ulden, 1986, J. Haz. Mat. 16, 411–425) are also included. In view of the limitations of the models, the agreement was good for both.

The equilibria of rotating isothermal clouds

view Abstract Citations (39) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Equilibria of Rotating Isothermal Clouds Kiguchi, Masayoshi ; Narita, Shinji ; Miyama, Shoken M. ; Hayashi, Chushiro Abstract The effect of rotation on the equilibrium structure of an axisymmetric isothermal gas cloud embedded in a stationary external medium is investigated by means of numerical simulations. The underlying assumptions, basic equations, input parameters, and numerical approach are explained, and the results are presented in extensive tables and graphs and characterized in detail. The critical central density and rotation energy, beyond which clouds become unstable to global contraction/expansion and ring formation, respectively, are found to be 800 times the boundary-surface density (BSD) and 0.44 times the gravitational energy. Stable rotating clouds are shown to have maximum mass 31 times that of nonrotating clouds, maximum mean rotation velocity 2.7 times the sound speed, and maximum mean density 6 times BSD. An expression for the maximum height of the boundary surface above the equatorial plane is derived. Publication: The Astrophysical Journal Pub Date: June 1987 DOI: 10.1086/165332 Bibcode: 1987ApJ...317..830K Keywords: Interstellar Gas; Isothermal Processes; Molecular Clouds; Nebulae; Rotating Fluids; Angular Momentum; Angular Velocity; Gas Density; Velocity Distribution; Astrophysics; INTERSTELLAR: MOLECULES; NEBULAE: GENERAL; ROTATION full text sources ADS |

A numerical method for three-dimensional simulations of collapsing, isothermal, magnetic gas clouds

In this paper a numerical method for the investigation of collapsing gas clouds will be described. The method is based on the particle method SPH (Smooth Particle Hydrodynamics). Different forms of the magnetic force terms are discussed and one form, that which conserves linear and angular momentum exactly, is shown to be unstable. A simple technique which removes the instability is described. The method is applied here to Alfvén waves, shock waves, magnetic polytropes and to equilibrium models of isothermal gas clouds.

Self-similar condensation of spherically symmetric self-gravitating isothermal gas clouds

The authors show that similarity solutions for the condensation of spherically symmetric self-gravitating isothermal gas clouds are much more abundant than was hitherto believed; they have discovered that each of the discrete solutions found by previous authors must be replaced by a bounded two-parameter continuum of such solutions. Moreover, the two parameters (z0, w0) admit a clear and useful physical interpretation. z0 characterizes the asymptotic form of a solution as t→-∞ (i.e. at very early times), and reflects how intrinsically unstable against contraction the cloud interior is from the outset. w0 characterizes the asymptotic form of a solution as t→+∞ (i.e. at very late times) and reflects how important external pressure is in driving compression. The evolution of the flow is presented and discussed.

Do we understand rotating isothermal collapses yet?

The scalar virial equation can be used to elucidate many interesting properties of equilibrium gas clouds when the effects of surface pressure, rotation, self-gravity, and internal isothermal gas pressure are considered simultaneously. Details regarding the internal structure of rotating isothermal gas clouds are ignored in order to obtain an analytical expression describing global cloud properties. Excellent agreement is obtained between the simple analytical model and other previously published, more detailed models in physical regimes where other models have been constructed. For the first time, a physical connection is drawn between the surface-pressure-dominated equilibrium models of S. W. Stahler (1983, Astrophys. J. 268, 155–184) and the rotation-dominated models of C. Hayashi, S. Narita, and S. M. Miyama (1982, Prog. Theor. Phys. 68, 1949–1966). Stable axisymmetric models of any mass and angular momentum can be constructed. Using the analytic expression for virial equilibrium as a foundation, all rotating, isothermal collapse calculations can now be well understood. Limiting properties of isothermal clouds are outlined, and realistic “starting” models for cloud collapse are proposed.

Analytic Solutions for Equilibrium of Rotating Isothermal Clouds: One-Parameter Family of Axisymmetric and Conformal Configurations

Equilibrium states of rotating axisymmetric isothermal gas clouds are studied and it is found that, if the linear rotation velocity is constant throughout a cloud, there exists a one-parameter family of conformal solutions which describe the equilibrium exactly. The parameter depends only on the ratio of the rotation velocity to the sound velocity and represents the flatness of equi-density contours in the direction of the rotation axis. Two extreme values of the parameter correspond to a non-rotating spherical configuration and a rapidly-rotating thin disk. Specific angular momentum on a cylindrical surface around the rotation axis is found to be proportional to the amount of mass contained inside the cylinder considered. The stability of the equilibrium configurations is examined analytically as well as numerically and it is found that nearly spherical configurations are unstable to contraction and expansion as a whole, very flat configurations are unstable to ring formation and intermediate configurations are likely to be stable. Finally, these equilibrium configurations are compared with the results of numerical computations which have so far been performed by many people on the collapse of rotating isothermal clouds.

Axisymmetric gravitational collapse of rotating interstellar gas clouds

Numerical calculations have been made of the gravitational axisymmetric collapse of isothermal gas clouds endowed with angular momentum. The evolutionary study is based on the so-called Fluid-in-Cell method coupled to an efficient algebraic algorithm which allows the Poisson equation to be integrated by means of block tri-diagonal matrices. The results, at ages slight larger than the initial free-fall time, indicate that flattened disk-shaped structures are formed in the central region of the clouds-in good agreement with the previous analytical results predicted by the authors.

Numerical three dimensional calculations of tidally induced binary protostar formation

The manner in which interstellar clouds fragment into subcondensations capable of becoming stars is still unknown. A variety of mechanisms may be responsible for cloud fragmentation. The considered investigation is concerned with a mechanism which is based on tidal forces due to a companion cloud. A 3D code is used to study the tidal effects of point sources of various masses located in the equatorial plane of an initially uniform density, uniformly rotating, isothermal gas cloud. It is found that tidal forces can be quite effective at inducing fragmentation in an initially uniform cloud, although the effects are most pronounced for low thermal energy clouds, perturbed by perhaps unreasonably large tidal forces. It is also seen that tidal forces may be much less efficient at preventing the collapse of a cloud than has previously been supposed.

Fragmentation of Isothermal Gas Clouds

An analysis of the collapse and fission of the isothermal gas cloud examined by Boss and Bodenheimer (BB, 1979) has been made using SPH (Lucy 1977, Gingold and Monaghan 1977, 1978). While SPH is not as effective as finite difference methods for problems with spherical symmetry, it has the advantage in fragmentation problems that the description of the fragments is independent of their position and orientation. Furthermore, the SPH algorithm has been tested by applying it to non axisymmetric problems for which accurate solutions are known as well as spherically and axially symmetric problems.

Fragmentation of rotating protostellar clouds

With a three-dimensional hydrodynamic computer code, the behavior of rotating, isothermal gas clouds as they collapse from Jeans unstable configurations is examined in order to determine whether they are susceptible to fragmentation during the initial dynamic collapse phase of evolution. It is found that a gas cloud will not fragment unless (1) it begins collapsing from a radius much smaller than the Jeans radius (i.e., the cloud initially encloses many Jeans masses) and (2) irregularities in the cloud's initial structure (specifically, density inhomogeneities) enclose more than one Jeans mass of material. Instead of fragmenting, most of the models collapse to a ring configuration. The rings appear to be less susceptible to fragmentation from arbitrary perturbations in their structure than has previously been indicated in other work. Because the models, which include the effects of gas pressure, do not readily fragment during a phase of dynamic collapse, it is suggested that gas clouds in the galactic disk undergo fragmentation only during quasi-equilibrium phases of their evolution.

Brightness and shape of radio jets

Observed radio structures of jets in radio galaxies are compared with models based on beam and slingshot theories. Simple beam models predict a rapid decline of surface brightness B with distance r from the nucleus, Bproportionalr/sup -m/, where m>3. This contrasts with the almost uniform brightness out to 200 kpc in the jet of NGC 6251 and out to 10 kpc in the northern jet of 3C 31. On the other hand, the slingshot model gives a good agreement with the structure of both jets if the radio galaxies are embedded in an isothermal gas cloud with core radius of approx.80kpc (NGC 6251) and approx.8 kpc (3 C 31).