Dense Gas Clouds Research Articles

CFD modeling of dense gas cloud dispersion over irregular terrain

Numerical calculations were performed to reproduce the transport and dispersion of the instantaneous release of finite volumes of dense gases over homogeneous flat, sloped, and complex terrain surfaces for calm and windy situations. The 1981 Porton Trial Test 8 field study of the release of 40m3 of a Freon-air mixture was used as a validation case to evaluate the behavior of volume integrated, cross-section integrated , depth integrated and full 3-d CFD models. In addition wind-tunnel measurements by Meroney & Lohmeyer (1983, 1984) about an instantaneous cloud of dense gas released in a wind were reproduced by similar calculations. All results were inter-compared, and time-dependent cloud dimensions, arrival times, and concentrations were considered. Very similar behavior was found for all models which tend to validate all methods as useful predictors.

Is molecular gas necessary for star formation?

On galactic scales, the surface density of star formation appears to be well correlated with the surface density of molecular gas. This has lead many authors to suggest that there exists a causal relationship between the chemical state of the gas and its ability to form stars -- in other words, the assumption that the gas must be molecular before star formation can occur. We test this hypothesis by modelling star formation within a dense cloud of gas with properties similar to a small molecular cloud using a series of different models of the chemistry, ranging from one in which the formation of molecules is not followed and the gas is assumed to remain atomic throughout, to one that tracks the formation of both H2 and CO. We find that presence of molecules in the gas has little effect on the ability of the gas to form stars: star formation can occur just as easily in atomic gas as in molecular gas. At low densities (< 10^4 cm^-3), the gas is able to cool via C+ fine-structure emission almost as efficiently as via CO rotational line emission, while at higher densities, the main cooling process involves the transfer of energy from gas to dust, meaning that the presence of molecules is again unimportant. Cooling by H2 is particularly inefficient, accounting for as little as 1 percent of the overall cooling in the cloud. Rather than the chemical makeup, we find that the most important factor controlling the rate of star formation is the ability of the gas to shield itself from the interstellar radiation field. As this is also a prerequisite for the survival of molecules within the gas, our results support a picture in which molecule formation and the formation of cold gas are both correlated with the column density of the cloud -- and thus its ability to shield itself -- rather than being directly correlated with each other.

A gas cloud on its way towards the supermassive black hole at the Galactic Centre

Measurements of stellar orbits provide compelling evidence that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares, Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low. Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud's orbit to be highly eccentric, with an innermost radius of approach of only ∼3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole's gravitational force. The cloud's dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.

Time-dependent CO2 variations in Lake Albano associated with seismic activity

Lake Albano (Alban Hills volcanic complex, Central Italy) is located in a densely populated area near Rome. The deep lake waters have significant dissolved CO2 concentrations, probably related to sub-lacustrine fluid discharges fed by a pressurized CO2-rich reservoir. The analytical results of geochemical surveys carried out in 1989–2010 highlight the episodes of CO2 removal from the lake. The total mass of dissolved CO2 decreased from ∼5.8 × 107 kg in 1989 to ∼0.5 × 107 kg in 2010, following an exponential decreasing trend. Calculated values of both dissolved inorganic carbon and CO2 concentrations along the vertical profile of the lake indicate that this decrease is caused by CO2 release from the epilimnion, at depth <9 m, combined with (1) water circulation at depth <95 m and (2) CO2 diffusion from the deeper lake layers. According to this model, Lake Albano was affected by a large CO2 input that coincided with the last important seismic swarm at Alban Hills in 1989, suggesting an intimate relationship between the addition of deep-originated CO2 to the lake and seismic activity. In the case of a CO2 degassing event of an order of magnitude larger than the one that occurred in 1989, the deepest part of Lake Albano would become CO2-saturated, resulting in conditions compatible with the occurrence of a gas outburst. These results reinforce the idea that a sudden CO2 input into the lake may cause the release of a dense gas cloud, presently representing the major volcanic threat for this densely populated area.

NUCLEAR STAR-FORMING RING OF THE MILKY WAY: SIMULATIONS

We present hydrodynamic simulations of gas clouds in the central kpc region of the Milky Way that is modeled with a three-dimensional bar potential. Our simulations consider realistic gas cooling and heating, star formation, and supernova feedback. A ring of dense gas clouds forms as a result of X1-X2 orbit transfer, and our potential model results in a ring radius of ~200 pc, which coincides with the extraordinary reservoir of dense molecular clouds in the inner bulge, the Central Molecular Zone (CMZ). The gas clouds accumulated in the CMZ can reach high enough densities to form stars, and with an appropriate choice of simulation parameters, we successfully reproduce the observed gas mass and the star formation rate (SFR) in the CMZ, ~2x10^7 Msun and ~0.1 Msun/yr. Star formation in our simulations takes place mostly in the outermost X2 orbits, and the SFR per unit surface area outside the CMZ is much lower. These facts suggest that the inner Galactic bulge may harbor a mild version of the nuclear star-forming rings seen in some external disk galaxies. Furthermore, from the relatively small size of the Milky Way's nuclear bulge, which is thought to be a result of sustained star formation in the CMZ, we infer that the Galactic inner bulge probably had a shallower density profile or stronger bar elongation in the past.

A NEW JEANS RESOLUTION CRITERION FOR (M)HD SIMULATIONS OF SELF-GRAVITATING GAS: APPLICATION TO MAGNETIC FIELD AMPLIFICATION BY GRAVITY-DRIVEN TURBULENCE

Cosmic structure formation is characterized by the complex interplay between gravity, turbulence, and magnetic fields. The processes by which gravitational energy is converted into turbulent and magnetic energies, however, remain poorly understood. Here, we show with high-resolution, adaptive-mesh simulations that MHD turbulence is efficiently driven by extracting energy from the gravitational potential during the collapse of a dense gas cloud. Compressible motions generated during the contraction are converted into solenoidal, turbulent motions, leading to a natural energy ratio of E_sol/E_tot of approximately 2/3. We find that the energy injection scale of gravity-driven turbulence is close to the local Jeans scale. If small seeds of the magnetic field are present, they are amplified exponentially fast via the small-scale dynamo process. The magnetic field grows most efficiently on the smallest scales, for which the stretching, twisting, and folding of field lines, and the turbulent vortices are sufficiently resolved. We find that this scale corresponds to about 30 grid cells in the simulations. We thus suggest a new minimum resolution criterion of 30 cells per Jeans length in (magneto)hydrodynamical simulations of self-gravitating gas, in order to resolve turbulence on the Jeans scale, and to capture minimum dynamo amplification of the magnetic field. Due to numerical diffusion, however, any existing simulation today can at best provide lower limits on the physical growth rates. We conclude that a small, initial magnetic field can grow to dynamically important strength on time scales significantly shorter than the free-fall time of the cloud.

Complexity and uncertainty in the assessment of the Viareggio LPG railway accident

The manuscript discusses the uncertainty and complexity intrinsic to the modeling of the accident occurred in Viareggio on June 29th, 2009 where a freight train loaded with LPG derailed and one of the tank cars released 45 t of LPG that finally exploded inside some houses and burnt. Thirty-one people died, thirty citizens were seriously injured and the damages were valued at 32 M€. The paper highlights the influence of the input variables on the accident consequences to determine the most probable sequence of events. Given the set of input data for the simulation of the accident, the most important of them were varied, within a suitable interval, to assess their influence on the accident consequences in terms of time the dense-gas cloud took to reach the farthest exploded house of Terminetto neighborhood ( i.e. about 100 m from the accident epicenter), and the pool-fire dimensions. The analysis reports the difficulties in applying quantitative models to real accidents, when the events do not comply with the model hypotheses. Nonetheless, the manuscript shows that it is possible to reconstruct the most probable sequence of events by investigating the more likely scenarios. The analysis shows that the assumption of a circular pool leads to the overestimation of the pool-fire height. In addition, the time of arrival of the dense-gas cloud does not present large variations among the different accident scenarios and it is always too short to allow implementing any mitigation actions or effective emergency plans.

Radio polarimetry of 3C 119, 3C 318, and 3C 343 at milliarcsecond resolution

<i>Aims. <i/>We report new Very Long Baseline Array (VLBA) polarimetric observations of the compact steep-spectrum (CSS) sources 3C 119, 3C 318, and 3C 343 at 5 and 8.4 GHz. <i>Methods. <i/>We analysed our VLBA observations and derived milliarcsecond-resolution images of the total intensity, polarisation, and rotation measure (<i>RM<i/>) distributions. <i>Results. <i/> The CSS source 3C 119, associated with a possible quasar, has source rest-frame <i>RM<i/> values up to ~10 200 rad m<sup>-2<sup/> in a region that coincides with a change in the direction of the inner jet. This component is located ~325 pc from the core, which is a variable source with a peaked radio spectrum. For 3C 318, which is associated with a galaxy, a rest-frame <i>RM<i/> of ~3030 rad m<sup>-2<sup/> was estimated for the brightest component contributing almost all of the polarised emission. Two more extended components were detected, that contain “wiggles” in the jet towards the southern side of the source. The CSS source 3C 343 contains two peaks of emission and a curved jet embedded in more diffuse emission. It exhibits complex field directions close to the emission peaks, which are indicative of rest-frame <i>RM<i/> values in excess of <i>≈<i/>6000 rad m<sup>-2<sup/>. The locations of the cores in 3C 318 and 3C 343 are unclear. <i>Conclusions. <i/>The available data about mas-scale rest-frame <i>RM<i/> estimates for CSS sources show that these have a wide range of values extending up to ~40 000 rad m<sup>-2<sup/> in the central region of OQ172, and may be located at projected distances from the core of up to ~1600 pc, as in 3C 43 where this feature has a rest-frame <i>RM<i/> of ~14 000 rad m<sup>-2<sup/>. The <i>RM<i/> estimates for the cores of core-dominated radio sources indicate that in addition to responding to an overall density gradient of the magneto-ionic medium, geometry, orientation, and modes of fuelling may also play a significant role. In addition to these effects, the high values of <i>RM<i/> in CSS sources are possibly caused by dense clouds of gas interacting with the radio jets. The observed distortions in the radio structures of many CSS sources are consistent with this interpretation.

The Viareggio LPG railway accident: Event reconstruction and modeling

This manuscript describes in detail the LPG accident occurred in Viareggio on June 2009 and its modeling. The accident investigation highlighted the uncertainty and complexity of assessing and modeling what happened in the congested environment close to the Viareggio railway station. Nonetheless, the analysis allowed comprehending the sequence of events, the way they influenced each other, and the different possible paths/evolutions. The paper describes suitable models for the quantitative assessment of the consequences of the most probable accidental dynamics and its outcomes. The main finding is that after about 80 s from the beginning of the release the dense-gas cloud reached the surrounding houses that were destroyed successively by internal explosions. This fact has two main implications. First, it shows that the adopted modeling framework can give a correct picture of what happened in Viareggio. Second, it confirms the need to develop effective mitigation measures because, in case of this kind of accidents, there is no time to apply any protective emergency plans/actions.

Cerenkov Line-like Radiation and the Origin of Broad Emission Lines in Quasars

When the thermal relativistic electrons with isotropic distribution of velocities move through a dense gas region or impinge upon the surface of a cloud of dense gas, the Cerenkov effect will produce peculiar atomic or ionic emission lines, which we call the “Cerenkov line-like radiation”. This prediction has been verified by the laboratory experiments in optical waveband. In this paper, the importance of the Cerenkov line-like radiation in the exploration of broad emission lines in quasars and Syf1 s is pointed out. By using this mechanism, some long standing and significant puzzles in the study of quasars could be solved. Furthermore, the magnitude orders of energy losses of various effects of a relativistic electron in cosmic gas are estimated and compared with each other to prove the effectiveness of this new mechanism in quasars.

Young and intermediate-age massive star clusters

An overview of our current understanding of the formation and evolution of star clusters is given, with the main emphasis on high-mass clusters. Clusters form deeply embedded within dense clouds of molecular gas. Left-over gas is cleared within a few million years and, depending on the efficiency of star formation, the clusters may disperse almost immediately or remain gravitationally bound. Current evidence suggests that a small percentage of star formation occurs in clusters that remain bound, although it is not yet clear whether this fraction is truly universal. Internal two-body relaxation and external shocks will lead to further, gradual dissolution on time scales of up to a few hundred million years for low-mass open clusters in the Milky Way, while the most massive clusters (>10(5) M(o)) have lifetimes comparable to or exceeding the age of the Universe. The low-mass end of the initial cluster mass function is well approximated by a power-law distribution, dN/dM proportional to M(-2), but there is mounting evidence that quiescent spiral discs form relatively few clusters with masses M > 2 x 10(5) M(o). In starburst galaxies and old globular cluster systems, this limit appears to be higher, at least several x10(6) M(o). The difference is likely related to the higher gas densities and pressures in starburst galaxies, which allow denser, more massive giant molecular clouds to form. Low-mass clusters may thus trace star formation quite universally, while the more long-lived, massive clusters appear to form preferentially in the context of violent star formation.

The abundance of HNCO and its use as a diagnostic of environment

We aim to investigate the chemistry and gas phase abundance of HNCO and the variation of the HNCO/CS abundance ratio as a diagnostic of the physics and chemistry in regions of massive star formation. A numerical-chemical model has been developed which self-consistently follows the chemical evolution of a hot core. The model comprises of two distinct stages. An initial collapse phase is immediately followed by an increase in temperature which represents the switch on of a central massive star and the subsequent evolution of the chemistry in a hot, dense gas cloud (the hot core). During the collapse phase, gas species are allowed to accrete on to grain surfaces where they can participate in further reactions. During the hot core phase surface species thermally desorb back in to the ambient gas and further chemical evolution takes place. For comparison, the chemical network was also used to model a simple dark cloud and photodissociation regions. Our investigation reveals that HNCO is inefficiently formed when only gas-phase formation pathways are considered in the chemical network with reaction rates consistent with existing laboratory data. Using currently measured gas phase reaction rates, obtaining the observed HNCO abundances requires its formation on grain surfaces. However our model shows that the gas phase HNCO in hot cores is not a simple direct product of the evaporation of grain mantles. We also show that the HNCO/CS abundance ratio varies as a function of time in hot cores and can match the range of values observed. This ratio is not unambiguously related to the ambient UV field as been suggested - our results are inconsistent with the hypothesis of Martin et al (2008). In addition, our results show that this ratio is extremely sensitive to the initial sulphur abundance.

A new Lagrangian particle model for the simulation of dense gas dispersion

A Lagrangian stochastic model (MicroSpray), able to simulate the airborne dispersion in complex terrain and in presence of obstacles, was modified to simulate the dispersion of dense gas clouds. This is accomplished by taking into account the following processes: negative buoyancy, gravity spreading and the particle's reflection at the bottom computational boundary. Elevated and ground level sources, continuous and instantaneous emissions, time varying sources, plumes with initial momentum (horizontal, vertical or oblique in any direction), plumes without initial momentum are considered. MicroSpray is part of the model system MSS, which also includes the diagnostic MicroSwift model for the reconstruction of the 3-D wind field in presence of obstacles and orography. To evaluate the MSS ability to simulate the dispersion of heavy gases, its simulation performances are compared in detail to two field experiments (Thorney Island and Kit Fox) and to a chlorine railway accident (Macdona). Then, a comprehensive analysis considering several experiments of the Modelers Data Archive is presented. The statistical analysis on the overall available data reveals that the performance of the new MicroSpray version for dense gas releases is generally reliable. For instance, the agreement between concentration predictions and observations is within a factor of two in the 72% up to 99% of the occurrences for the case studies considered. The values of other performance measures, such as correlation coefficient, geometric mean bias and geometric variance, mostly set in the ranges indicated as good-model performances in the specialized literature.

Ejection and descent of matter in the solar atmosphere

The descent and ejection of matter in the solar atmosphere observed in the CaII 8498-A line have been studied. In the NOAA active region no. 10 792 on July 30, 2005 before the flare, the dense cold gas cloud descended with a ray velocity of ∼8 km/s and then ascended in the impulsive phase. The plasma ascended with an acceleration reaching 0.4 km/s2 in the flare maximum. The acceleration of the matter likely continued after the flare maximum, because an acceleration of higher than 0.5 km/s2 was required for the appearance of the ejection at the edge of the occulting disk of the LASCO C2 coronagraph at 0557 UT. The descent of the matter resulting in the local heating of the chromosphere was also observed in the NOAA active region no. 10656 on August 9, 2004 before the flare. The maximum descent velocity was no more than 24.7 km/s.

Is the PAMELA anomaly caused by supernova explosions near the Earth?

We show that the anomaly of the positron fraction observed by the PAMELA experiment can be attributed to recent supernova explosion(s) in a dense gas cloud (DC) near the Earth. Protons are accelerated around the supernova remnant (SNR). Electrons and positrons are created through hadronic interactions inside the DC. Their spectrum is harder than that of the background because the SNR spends much time in a radiative phase. Our scenario predicts that the anti-proton flux dominates that of the background for >~100 GeV. We compare the results with observations (Fermi, HESS, PPB-BETS, and ATIC).

Star formation branches out

Deciphering how stars form within turbulent, dense clouds of molecular gas has been a challenge. An innovative technique that uses a tree diagram provides insight into the process.

CFD model simulation of dispersion from chlorine railcar releases in industrial and urban areas

To assist in emergency response decisions and planning in case of releases of pressurized liquefied chlorine from railroad tank cars in industrial sites and cities, the FLACS Computational Fluid Dynamics (CFD) model has been used to simulate the transport and dispersion of the dense chlorine cloud. Two accident locations are studied: an actual railcar accident at an industrial site in Festus, MO, and a hypothetical railcar accident at a rail junction in the Chicago urban area. The results show that transport of a large dense gas release at ground level in an industrial site or large city could initially extend a hundred meters or more in the upwind and crosswind directions. The dense cloud may follow terrain drainage, such as river channels. Near the source, the obstacles tend to slow down the dense gas cloud and may constrain it and cause increased concentrations. Farther downwind, the obstacles may cause enhanced mixing and dilution once the cloud has grown larger. In some cases, significant amounts of cloud mass may become “trapped” in obstacle wakes for many minutes after the main cloud has passed. Although the CFD model can account for the details of the flow and dispersion much better than standard widely-used simple dense gas models, many similarities are found among the various models in their simulated variations with downwind distance of the maximum cloud centerline concentration.

The role of deposition in limiting the hazard extent of dense-gas plumes

Accidents that involve large (multi-ton) releases of toxic industrial chemicals and form dense-gas clouds often yield far fewer fatalities, casualties and environmental effects than standard assessment and emergency response models predict. This modeling study, which considers both dense-gas turbulence suppression and deposition to environmental objects (e.g. buildings), demonstrates that dry deposition to environmental objects may play a significant role in reducing the distance at which adverse impacts occur—particularly under low-wind, stable atmospheric conditions which are often considered to be the worst-case scenario for these types of releases. The degree to which the released chemical sticks to (or reacts with) environmental surfaces is likely a key parameter controlling hazard extents. In all modeled cases, the deposition to vertical surfaces of environmental objects (e.g. building walls) was more efficient in reducing atmospheric chemical concentrations than deposition to the earth’s surface. This study suggests that (1) hazard extents may vary widely by release environment (e.g. grasslands vs. suburbia) and release conditions (e.g. sunlight or humidity may change the rate at which chemicals react with a surface) and (2) greenbelts (or similar structures) may dramatically reduce the impacts of large-scale releases. While these results are demonstrated to be qualitatively consistent with the downwind extent of vegetation damage in two chlorine releases, critical knowledge gaps exist and this study provides recommendations for additional experimental studies.

IRAC Observations of COJ= 4 → 3 High‐Velocity Cloud in the 30 Doradus Complex in the Large Magellanic Cloud

We present the results of 12CO J = 2 → 1 observations of the X-ray-bright giant shell complex 30 Doradus in the Large Magellanic Cloud (LMC) using the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO). This is the one of the largest H II complexes in the Local Group. We compare the 12CO J = 2 → 1 observations against previously made 12CO J = 4 → 3 observations and analyze the spatial distribution of young stellar objects (YSOs) within the cloud using the Spitzer IRAC observations of the 30 Doradus complex. Both peaks of 12CO J = 2 → 1 and J = 4 → 3 emitting clouds coincide with the densest region of the filaments in which multiple shells are colliding. The YSOs are clustered in the southern ridge of the warm and dense molecular gas clouds traced by 12CO J = 4 → 3, indicating a filamentary structure of star formation throughout 30 Doradus. We also find an excess of Class I YSO candidates close to the clouds, which likely represent the most recent phase of star formation in this region. This is a region where the triggered star formation has actually occurred, and newly formed stars may have produced such a high-velocity outflow through interacting with the surrounding molecular cloud material.

Super Star Cluster NGC 1705‐1: A Local Analog to the Birth Site of Long‐Duration γ‐Ray Bursts

Recent observations suggest that global properties of the host galaxies for long-duration γ-ray bursts (GRBs) are particularly well suited for creating the massive star progenitors of these GRBs. Motivated by the hypothesis that massive young star clusters located in metal-poor, low-metallicity galaxies are a natural birth site for GRB progenitors, we present a comparison study of the ISM properties along the sight line toward the super star cluster NGC 1705-1 and those in distant GRB hosts. Using the same set of metal transitions in the UV and applying known ISM structures in NGC 1705, we find that NGC 1705-1 resembles distant GRB host galaxies in its high neutral gas column, low molecular gas fraction, low metallicity, α-element enhancement, and low dust depletion. The lack of molecular gas is due to the enhanced UV radiation field in the starburst environment, consistent with the expectations for GRB progenitors. In addition, the known presence of dense neutral gas clouds at r 500 pc from NGC 1705-1 provides a natural reservoir of C+, Si+, and Fe+ ions that may be subsequently excited by the afterglow UV radiation field to produce excited lines commonly seen in GRB host ISM. We further argue that the apparent offset in the velocity profiles of low- and high-ionization transitions from absorption-line observations alone already offers important clues for related starburst episodes in GRB host galaxies. Our study shows that a statistical comparison between the ISM around star clusters and high-redshift GRB progenitors is important for identifying the key physical parameters that facilitate the formation of GRBs.