Diffuse Gas Clouds Research Articles

Preparation of CO2/N2 cryogenic slurry and its pipeline flow characteristics

It was proposed to use CO2/N2 to effectively prevent and control Class A solid spontaneous fires such as coal. The preparation process of CO2/N2 cryogenic slurry pipeline and its pipeline flow characteristics were studied. The temperature field, pressure field and phase change characteristic changed during the transportation of the CO2/N2 mixture in the pipeline are analyzed by numerical simulation. The results showed that: the fluid in the pipeline exchanged heat and started to contact at a position of 0.1m in the pipeline. The temperature first rapidly dropped to the lowest temperature of 103K, and then the fluids in the range of 0.1m to 0.3m were fully mixed, and the temperature rised to 164K. The temperature was consistent with the equilibrium state in the pipe, and the temperature was gentle and stable; the pressure in the pipe was on the whole downward trend, and each position along the main pipe produced a positive pressure difference relative to the end of the pipe. The closer to the mixing place, the greater the corresponding pressure difference; During the mixing process, LN2 was acted as a cold source to sublime CO2 to form dry ice particles. And through the cryogenic fluid mixing experiment, the temperature distribution in the pipeline, range of the dry ice particle generation position, and the dry ice particle generation phenomenon was measured. The results showed that the mixing was completed within 0.8 m of the pipeline, and the temperature in the tube rised to 168K and remain stable. The data were basically consistent. And dry ice particles generated at the temperature of 194.5K, and a diffuse gas cloud formed at the outlet with the momentum of the jet.

3D AMR simulations of the evolution of the diffuse gas cloud G2 in the Galactic Centre

AbstractWith the help of 3D AMR hydrodynamical simulations we aim at understanding G2’s nature, recent evolution and fate in the coming years. By exploring the possible parameter space of the diffuse cloud scenario, we find that a starting point within the disc of young stars is favoured by the observations, which may hint at G2 being the result of stellar wind interactions.

Hydrodynamical simulations of G2 interpreted as a diffuse gas cloud

AbstractRecently the gas and dust cloud “G2” was discovered on a highly eccentric orbit around the massive black hole in the Galactic center. The orbit will bring the cloud as close as 2400 Schwarzschild radii to Sgr A* beginning of 2014. With the help of hydrodynamical simulations using the PLUTO code, we investigate possible origins and the fate of the cloud in the coming years. In this proceedings article, we concentrate on a scenario where G2 is interpreted as a diffuse gas cloud and show its detailed evolution in the observable position-velocity diagrams. We further elaborate on the problem of the tail emission which might or might not be related to the G2 cloud.

Multiwavelength observations of cirrus clouds in the North Celestial Loop: a study of the OH emission

New 1665- and 1667-MHz OH observations were made with the Green Bank Telescope at an angular resolution of 7 arcmin of 108 locations over two diffuse gas clouds at high Galactic latitude. A linear correlation is observed between the IRAS 100 μm infrared intensity, which traces all the gas, and the OH column density, indicating that the amount of OH increases with the visual extinction above a threshold value of Av ≈ 0.5. N(OH) increases monotonically with N(H I) in up to N(OH) ≈ 0.25-0.3 x 10 14 cm -2 . At greater values of N(OH) the H I column density saturates at N(H I) ≈ 5 x 10 20 cm -2 , suggesting that there is molecular gas not traced by H I. No correlation is found between the 12 CO integrated intensity and the OH column density, indicating that OH might be a better tracer of H 2 than CO in those diffuse regions. An H I column density of at least ≈2.2-2.9 x 10 20 cm -2 is needed for the formation of OH, but in regions where the cloud is less shielded from the interstellar radiation field the value rises to N(H I) = 3.3 x 10 20 cm -2 .

Supersonic cloud collision – II

In this second paper of the sequel of two papers, we further investigate the problem of molecular cloud (MC) collision. Anathpindika (2009) (hereafter paper I) considered highly supersonic cloud collisions and examined the effect of bending and shearing instabilities on the shocked gas slab. We now consider moderately supersonic cloud collisions (precollision cloud velocities of order 1.2 km s$^{-1}$ to 2.4 km s$^{-1}$). In the current paper, we present five SPH simulations of fast head-on and/or off-centre cloud collisions to study the evolution of ram pressure confined gas slabs. The relevant thermodynamics in the problem is simplified by adopting a simple barytropic equation of state. We explore the parameter space by varying the pre-collision velocity and the temperature of the post collision gas slab. The temperature in a pressure compressed gas slab is crucial to its dynamical evolution. The pressure confined gas slabs become Jeans unstable if the average sound crossing time, $t_{cr}$, of putative clumps condensing out of them, is much larger than their free fall time, $t_{ff}$. Self gravitating clumps may spawn multiple/larger $N$-body star clusters. Warmer gas slabs are unlikely to fragment and may end up as diffuse gas clouds.

Microlensing by compact objects associated with gas clouds

We investigate gravitational microlensing of point-like lenses surrounded by diffuse gas clouds. Besides gravitational bending, one must also consider refraction and absorption phenomena. According to the cloud density, the light curves may suffer small to large deviations from Paczyński curves, up to complete eclipses. Moreover, the presence of the cloud endows this type of microlensing event with a high chromaticity and absorption lines recognizable by spectral analysis. It is possible that these objects populate the halo of our galaxy, giving a conspicuous contribution to the fraction of the baryonic dark matter. The required features for the extension and the mass of the cloud to provide appreciable signatures are also met by several astrophysical objects.

Dissipative processes in galaxy formation.

A galaxy commences its life in a diffuse gas cloud that evolves into a predominantly stellar aggregation. Considerable dissipation of gravitational binding energy occurs during this transition. I review here the dissipative processes that determine the critical scales of luminous galaxies and the generation of their morphology. The universal scaling relations for spirals and ellipticals are shown to be sensitive to the history of star formation. Semiphenomenological expressions are given for star-formation rates in protogalaxies and in starbursts. Implications are described for elliptical galaxy formation and for the evolution of disk galaxies.

Star formation - Phase transition, not Jeans instability

view Abstract Citations (24) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star formation - Phase transition, not Jeans instability Tohline, J. E. Abstract The use of second-order tensor virial equations to obtain analytical descriptions of rotating isothermal equilibrium sequences is reviewed, and their significance when dynamical instabilities are taken into account is considered. It is shown that along every rotating, isothermal, equilibrium sequence that permits a Jeans-type instability, a nondynamical phase transition to the higher-density, disk-like equilibrium sequence is also available. The transition changes a diffuse gas cloud into a compact structure closer to the stellar state, and occurs at masses smaller than the Jeans mass. The ratio of the phase transition mass to the Jeans mass is very sensitive to the specific angular momentum contained in a diffuse cloud. The existence of such an allowed phase transition in diffuse gas clouds opens up the possibility that star formation may not always proceed by a dynamical transition. Publication: The Astrophysical Journal Pub Date: May 1985 DOI: 10.1086/163144 Bibcode: 1985ApJ...292..181T Keywords: Computational Astrophysics; Interstellar Gas; Jeans Theory; Molecular Clouds; Phase Transformations; Star Formation; Angular Velocity; Gibbs Free Energy; Magnetohydrodynamics; Milky Way Galaxy; Rotating Environments; Astrophysics full text sources ADS |

Theoretical Processes in Star Formation

To bring out the basic problem of star formation, consider a non-rotating cosmical gas cloud which is free to contract and so to convert most of its gravitational energy into kinetic energy of mass motion. Fluid dynamical systems in general tend to dissipate their kinetic energy, and a diffuse gas cloud easily radiates excess heat. If the cloud did not break up into fragments, then the steady conversion of gravitational energy into radiation would lead ultimately into the formation of a compact massive body - perhaps a quasar - but not of a star cluster, which is maintained at a comparatively low mean density by random stellar kinetic energy. Thus the transition from a gravitationally-bound cloud of gas and dust into a bound star cluster - or a fortiori into an expanding O-B association - requires that gravitational energy released during collapse be conserved as the random macroscopic kinetic energy. It is very likely that star formation is often triggered by processes that are essentially dissipative, such as shocks following passage of the spiral wave, or cloud-cloud collisions; however, it is equally important to emphasise that we require at some stage a cut-off in kinetic energy dissipation. And indeed, once a cloud has been able to fragment into a group of self-gravitating blobs, each of small geometrical cross-section, the chance of inelastic collisions between blobs is sharply reduced.

Cosmic Origin of Radiation at Radio Frequencies

THE recent identification by W. Baade and R. Minkowski of the intense discrete radio source in Cygnus as being an emission from two colliding galaxies1 raises a number of interesting theoretical questions. In the first place, the only material likely to be seriously disturbed by such a collision is the diffuse interstellar material lying between the stars in these galaxies. Consequently, it would seem profitable to seek a process of generation of radio waves that takes place in extremely diffuse gas clouds. Secondly, processes of magnetic resonance requiring field intensities of many gauss, such as the Larmor precession, would seem to be excluded, since fields of this intensity could scarcely be maintained throughout regions of galactic dimensions.