Dense Gas Regions Research Articles

Thermodynamic evolution of the cosmological baryonic gas

The problem of galaxy formation and its dependence on thermodynamic properties is addressed by using Eulerian hydrodynamic numerical simulations of large scale structure formation. Global galaxy properties are explored in simulations including gravitation, shock heating and cooling processes, and following self-consistently the chemical evolution of a primor- dial composition hydrogen-helium plasma without assuming collisional ionization equilibrium. The galaxy formation model is mainly based on the identification of converging dense cold gas regions. We show that the evolution at low redshift of the ob- served cosmic star formation rate density is reproduced, and that the galaxy-like object mass function is dominated by low-mass objects. The galaxy mass functions are well described by a two power-law Schechter function whose parameters are in good agreement with observational fits of the galaxy luminosity function. The high-mass end of the galaxy mass function includes objects formed at early epochs and residing in high-mass dark matter halos whereas the low-mass end includes galaxies formed at later epochs and active in their stellar mass formation. Finally, the influence of two other physical processes, photoion- ization and non-equipartition processes between electrons, ions and neutrals of the cosmological plasma is discussed and the modifications on galaxy formation are examined.

Complex Molecules in the Hot Core of the Low‐Mass Protostar NGC 1333 IRAS 4A

We report the detection of complex molecules (HCOOCH3, HCOOH, and CH3CN), signposts of a hot core-like region, toward the low-mass Class 0 source NGC 1333 IRAS 4A. This is the second low-mass protostar in which such complex molecules have been searched for and reported, the other source being IRAS 16293-2422. It is therefore likely that compact (a few tens of AU) regions of dense and warm gas, where the chemistry is dominated by the evaporation of grain mantles and where complex molecules are found, are common in low-mass Class 0 sources. Given that the chemical formation timescale is much shorter than the gas hot-core crossing time, it is not clear whether the reported complex molecules are formed on the grain surfaces (first-generation molecules) or in the warm gas by reactions involving the evaporated mantle constituents (second-generation molecules). We do not find evidence for large differences in the molecular abundances, normalized to the formaldehyde abundance, between the two solar-type protostars, suggesting perhaps a common origin.

Mix experiments using a two-dimensional convergent shock-tube

This article reports the first Richtmyer–Meshkov instability experiments using an improved version of the Atomic Weapons Establishment convergent shock tube. These investigate the shock-induced turbulent mixing across the interfaces of an air/dense gas/air region. Multipoint ignition of a detonatable gas mixture produces a cylindrically convergent shock that travels into a test cell containing the dense gas region. The mixing process is imaged with shadowgraphy. Sample results are presented from an unperturbed experiment and one with a notch perturbation imposed on one of the dense gas interfaces. The unperturbed experiment shows the mixing across the dense gas boundaries and the motion of the bulk dense gas region. Imposition of the notch perturbation produces a mushroom-shaped air void penetrating the dense gas region. Three-dimensional simulations performed using the AWE TURMOIL3D code are presented and compared with the sample experimental results. A very good agreement is demonstrated. Conducting these first turbulent mixing experiments has highlighted a number of areas for future development of the convergent shock-tube facility; these are also presented.

Optical afterglow of the γ-ray burst of 14 December 1997

The very recent detection of the faint host galaxy of one γ-ray burst1,2,3,4 and the determination of a cosmological redshift for another5, demonstrates that these events are the most luminous phenomena in the Universe, emitting more energy in radiation than a supernova over just a few seconds. The source of this energy is still unknown, but may become clear through studies of the counterparts at longer wavelengths. Here we report the detection of an optical counterpart to a γ-ray burst (GRB971214) that occurred on 14 December 1997. It faded rapidly over a two-week period, just like the previous two optical transients1,6,7,8,9,10,11 which dispels any doubt that the three events are the optical afterglows of γ-ray bursts. The 14 December optical transient is the faintest of the three, and also is much redder than the other two. This reddening probably arises because of scattering by interstellar dust along the line of sight, which is presumably present in the denser regions of the host galaxy, where stars form. This suggests that the burst's progenitor did not stray too far from the point of its birth, which, regardless of the nature of the source, appears to be in a region of dense gas.

OH Satellite-Line Masers in the Nucleus of NGC 253

We report the detection of 1720 and 1612 MHz OH ground-state satellite-line maser regions in the starburst galaxy NGC 253. We find 10 compact maser features, all of which are located along the narrow ridge of bright radio continuum emission within the central 10 arcsec2 of the nucleus. The compact OH features appear physically associated with regions of dense molecular gas, supernova remnants, and H II regions. The data for several of the features are consistent with collisional inversion from the interaction of supernova remnant shocks with molecular gas, similar to that found for Galactic supernova remnants by Frail et al. For many other features near the nucleus of NGC 253, we find conjugate behavior in the 1720 and 1612 MHz lines, with one line showing stimulated emission and the other showing stimulated absorption. These results suggest the importance of infrared pumping associated with the active star formation in NGC 253 and are consistent with large column densities of molecular gas [N(H2) 1022 cm-2] .

HSTand Merlin Observations of 3C 264—A Laboratory for Jet Physics and Unified Schemes

We present new HST optical continuum and emission line WFPC2 images and MERLIN radio observations of 3C 264 at ~01 resolution. The jet is well resolved in both the optical and radio images. In addition, we report the discovery of an apparent optical at a projected radius of ~300-400 pc. The ring is most likely the manifestation of absorption by a nearly face-on circumnuclear dust disk. We discuss the evolution of the jet properties with distance. The jet collimation, brightness, and orientation change dramatically as it crosses the outer boundary of the suggesting an interaction between the jet and dense circumnuclear gas. We present a model for the jet propagation in which an initially relativistic jet decelerates as it crosses through a region of dense cold gas in the inner region of the galaxy. We derive the equations for brightness variations along an adiabatically expanding relativistic jet, and we model the jet brightness in 3C 264 as the combined effects of Doppler boosting, and adiabatic losses as traced through the jet velocity and width. We find that the data are consistent with a model in which the jet is initially highly relativistic (v ~ 0.98c, γ = 5) and we view it at roughly 50° inclination. We suggest that 3C 264 may serve as a laboratory for the study of relativistic entraining jets and may help us to understand the deceleration of jets, which is required in unifying schemes for FRI radio galaxies and BL Lac objects.

Induced star formation in a radio lobe of 3C 285?

view Abstract Citations (83) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Induced Star Formation in a Radio Lobe of 3C 285? van Breugel, Wil J. M. ; Dey, Arjun Abstract We report the discovery of a blue, star-forming region in the eastern lobe of the nearby (z = 0.0794), classical double-lobed radio galaxy 3C 285. This region is elongated along the radio source axis, and its optical line emission, and possibly its optical continuum emission, appear to be edge-brightened on the side facing the nucleus of 3C 285. We propose that the star formation in this region was triggered by the passage of the radio lobe through a region of dense gas in the intergalactic medium. Our observations add support to models in which powerful radio sources induce star formation through the compression of dense ambient material and are also in agreement with such an interpretation for the alignment effect observed in high-redshift radio galaxies. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173103 Bibcode: 1993ApJ...414..563V Keywords: H Ii Regions; Radio Galaxies; Star Formation; Astronomical Spectroscopy; Intergalactic Media; Interstellar Matter; Polarized Light; Radio Jets (Astronomy); Red Shift; Astrophysics; RADIO CONTINUUM: GALAXIES; GALAXIES: INDIVIDUAL ALPHANUMERIC: 3C 285; STARS: FORMATION full text sources ADS | data products SIMBAD (1) NED (1)

A method for introducing scaling behaviour into a classical equation of state

Equilibrium properties of fluids calculated from classical equations of state are inaccurate near the critical point. Fox has suggested a method of transforming the coordinates of a classical equation to non-classical coordinates so as to improve the fit to critical region properties. We have applied Fox's method to the cubic equation of state proposed by Patel and Teja, and have examined the improvement in the fit to near-critical (p, Vm, T) isotherms of carbon dioxide, methane, ethene and propane. In the liquid, near-critical, and dense gas regions the improvement is substantial.

Density effects on ion mobilities in electron attaching fluids: CS2, SF6, and C6F6

Ion mobilities μ were measured at densities n ranging from those of the low density gas (n < 0.01 nc, where nc = critical fluid density) through the dense gas and low-density liquid transition region (~0.5nc to 2nc) to the normal liquid (~3nc). The mobility at a given density was constant over a 10-fold variation of electric field strength, typically in the range 0.1 < E(MV/m) < 5. In the gas phase changes in mobility were dominated by changes in gas density. Comparison with neutral molecule transport in SF6 was used to illustrate the change in the effect of the ion on its local environment from that of molecular cluster formation to that of liquid electrostriction as density was increased. In the liquid, changes in ion mobility are dominated by changes in free volume. Mobilities in the present liquids could be described without reference to the activation energies suggested by the Arrhenius model. Keywords: ion, mobility, fluid, density, electron attaching.

Quantum hall effect in wide parabolic GaAs/Al xGa 1−xAs wells

Parabolic well structures have been proposed to produce thick spatially uniform regions of dense high mobility electron gas. Wide parabolic GaAs/Al xGa 1−xAs wells were fabricated using molecular beam epitaxy by grading the average Al concentration. These structures have high mobility at low temperatures (μ = 5 × 10 4 to 2.5 × 10 5 cm 2/V sec) and show the integer and fractional quantum Hall effect. Characteristic changes in the integer quantum Hall effect and the longitudinal magnetoresistance in a 4000 Å wide well are measured as the well is partially filled with electrons using persistent photoconductivity. These data establish that up to three electric subbands of the well are occupied.

Transport properties of polydisperse fluids. I. Shear viscosity of polydisperse hard-sphere fluids

The shear viscosity expressions for a polydisperse hard-sphere fluid in both the dilute and dense fluid regions have been obtained as the solution of a set of linear integral equations in the Enskog transport theory. In the monodisperse limit we recover the known Enskog result. Explicit analytical solutions have been obtained for the special case of equal-mass particles. For general mass-size distributions, a simple numerical method has been proposed. Computations have been performed for a mass-size relation of power-law form. The two size distributions we have looked at are the Schulz distribution and the log-normal distribution. It is found that both distributions show similar effects on the shear viscosity. In the dilute-gas region, for each given fractal dimension, there exists a critical variance of the size distribution at which the shear viscosity attains a maximum. In the dense-gas region, the shear viscosity increases as the variance and the density increase except for the low density and the high fractal-dimension region where the shear viscosity attains a minimum at some particular variance. The shear viscosity diverges at a close-packing density that decreases with increasing variance.

Diffusion of Liquid Hydrocarbons in Supercritical CO2 by Photon Correlation Spectroscopy

AbstractBinary diffusion coefficients of benzene and heptane in supercritical CO2 were measured by photon correlation spectroscopy. The measurements were carried out along isochores in the dense gas region at concentrations less than 5 mole percent hydrocarbon. The results of an analysis of data using mode‐mode‐coupling theory indicated that the exponent of the correlation length, ν = 0.67 ± 0.03 on the critical isochore. On the other hand, ν = 0.86 ± 0.05 was obtained on isochores that were not critical and the background contribution became significant.

A new perturbation expansion for fluids of nonspherical molecules

The free energy of a fluid composed of nonspherical molecules is expanded about that for a reference fluid of rigid, nonspherical molecules. The expansion procedure involved separating the intermolecular pair potential into repulsive and attractive regions for each set of relative orientations, and is similar to that used by Weeks, Chandler, and Andersen for monatomic fluids. The properties of the reference system are evaluated using the expansion of Bellemans. Calculated thermodynamic properties are compared with Monte Carlo results for fluids with a potential uLJ + ua, where uLJ is the Lennard-Jones model and ua is either the point dipole, point quadrupole, or an anisotropic overlap potential. For dipoles, agreement is satisfactory for values of the reduced dipole moment μ/(εσ3)1/2 up to about 1, while for quadrupoles good agreement is found for Q/(εσ5)1/2 up to about 0.8. For larger multipole moments the Bellemans expansion fails. For anisotropic overlap potentials the method seemed satisfactory for all values of anisotropic strength tested. A simplified form of the perturbation expansion is also obtained by introducing a generalized van der Waals model. The resulting equation of state is tested against data for CO2 in the dense gas and liquid regions, including the vapor–liquid equilibrium region. The functional form of this equation gives an accurate representation of the data for all temperatures and densities except those in the critical region, and also for the low temperature dilute gas.

Three Atom Correlations in the Lennard-Jones Fluid

The Monte Carlo method has been used to compute the triplet correlation function in a classical fluid with Lennard-Jones interactions. The computations were performed for particular configurations at five thermodynamic states of high density. The structure of the triplet function is discussed in the liquid and dense gas regions. Several closure approximations, which express the triplet function in terms of the pair correlation function, are compared to the Monte Carlo results.

Friction coefficients and self‐diffusivity reduced state correlations and interrelationship

AbstractTheretical equations for the transport properties of donse gases and liquids require knowledge of the contact pair‐correlation function g2 (σ) and the friction coefficient ζ. Usually this latter quantity is made up of a hard‐core contribution ζH and a soft‐repulsion contribution ζS. Correlations are presented for the reduced friction coefficients ζrH, ζrS, and ζr in terms of reduced density ρ* and reduced temperature T*. These correlations were found to predict the dependence of self‐diffusivity of several substances on temeratures and density in the dense gas region.

Depolarization of light scattered by systems of anisotropic molecules

A theoretical expression is derived for the depolarization factor of the Rayleigh light scattered by a system of anisotropic molecules, which have a center of inversion, in the dense gas region. This expression is a generalization of previous theories in that it contains besides the anisotropy arising from the structure of the molecules also anisotropy effects coming from the mutual orientation and from the induced dipole interaction. A qualitative discussion of the results leads to the conclusion that the depolarization factor is finite in the critical region. Numerical calculations have been carried out for CO 2; the results are compared with those of the existing theories and with provisional experimental data.

Recalculation of the friction constant and transport coefficients of liquid argon from the Rice-Allnatt theory

The linear trajectory approximation of the ?soft? friction constant in the Rice-Allnatt theory of transport has been computed with specific attention to the lower limit of the integral. The results are significantly different from the Palyvos-Davis values for ζS in the dense gas region but agree within 2% in the liquid region. The Rice- Allnatt expressions for the coefficients of shear viscosity and thermal conductivity have been simplified and a correction of a numerical error in the collisional contributions to momentum and heat transfer is made. The coefficients D, η, and λ have been calculated for the corrected ζS and related expressions. No significant change in D is obtained, but a worsening of agreement with experimental viscosities and thermal conductivities occurs. Conversely, a better prediction of the ratio mλ/kη is obtained. More recent viscosity data for liquid argon indicate the theory is less satisfactory than has previously been considered. These results suggest that any improvement of this class of theory can only come through the use of a better representation of the radial distribution function.

Relaxation Times for the Volume Viscosity of Simple Fluids

Effective relaxation times for the volume viscosity of a corresponding-states fluid in the dense-gas region are calculated using low-frequency volume viscosity data and previously developed estimates of high-frequency elastic moduli. The times are found to be on the order of 10−13 sec.

Hole Theory of Liquids and Dense Gases. II. Internal Entropy, Energy, and Heat Capacity

The internal entropy, energy, and heat capacity of the inert gases in the liquid and dense gas states are calculated on the basis on the hole theory of liquids under the assumptions that the free volume increases linearly with the number of neighboring holes and that the holes and molecules are randomly distributed. The agreement with experiment is good in the dense gas region but becomes less satisfactory for the liquid state. The various assumptions inherent in this work are discussed and improvements are suggested.

Predicting the Viscosity of Pure Light Hydrocarbons

Abstract The following equation, which describes the viscosity of methane, ethane, propane and n-butane in the vapor, liquid and dense-fluid regions for densities up to 2.4 times the critical density, is presented. The atmospheric-pressure viscosity can be reprevented satisfactorily by Sutherland's equation for which values of the necessary constants are given. The equation represents the data on these materials over the entire region with a standard deviation of 1.6 per cent for 288 points. Except in the immediate vicinity of the critical density, the largest difference between predicted and observed viscosity was 4.3 per cent. To facilitate calculations, the equation is presented as a single curve of, evaluated for a gas of zero molecular weight. By modification of the co-ordinates, the curve becomes a straight line. The factor for converting the curve value of to that for the actual gas is a linear function of molecular weight, and is also plotted. Introduction The pressures at which fluids are produced, transferred and processed have increased steadily in the petroleum and chemical industries. This has resulted in increased interest in the effect of pressure on the thermodynamic and transport properties of fluids. The relationships derived from simple kinetic theory often may be applied in estimating gas properties for low and moderate pressures. These have the advantage of simplicity, a fact which has frequently led to use beyond the range of proper applicability. At high pressures and low temperatures, these relationships may be greatly in error, and other means of calculation are needed for the dense gas and liquid regions. The thermodynamic properties of fluids have been studied extensively, both theoretically and experimentally. The volumetric behavior of a large number of fluids has been measured experimentally to high pressures for wide ranges of temperature. It is more difficult, however, to obtain accurate experimental values for transport properties, and detailed data have been obtained for very few fluids for extensive ranges of temperature and pressure. This situation has greatly handicapped correlation efforts. Because of the limited data available on transport properties and the complex relationships which exist between the transport and thermodynamic properties, generally one of three methods has been applied to represent these properties for pure fluids.Tabulations of each transport property at selected pressure andtemperature intervals.Equations for each transport property of each fluid which relatethese properties to PVT behavior.Generalized co-ordinate chart for each transport property, frequently with serious restrictions as to accuracy. The methods are listed in order of decreasing accuracy. The first and second methods are limited to pure components and certain commonly occurring mixtures such as air for conditions other than atmospheric pressure. Equations have been developed from kinetic theory which quite accurately represent the temperature dependence of viscosity of gases and liquids at low pressures. Special equations have been developed to calculate the effects of pressure and temperature on viscosity of steam and nitrogen, but these equations are empirical and different for each fluid. No single equation is presenting available for accurate prediction of viscosity in both the liquid and gas phases, for any fluid. The third method is based on van der Waals' theory, of corresponding states. Uyehara and Watson presented a plot of vs, with lines of constant which is generally accurate within 10 per cent, but in the critical region errors may be as large as 30 per cent. Carr and Comings, Mayland and Egly used, and as parameters and developed generalized correlations for gases, and Carr extended his to include mixtures. For natural gases Carr's chart is generally accurate within 3 per cent, but is less accurate for heavier hydrocarbons or other gases. The purpose of this study was to examine the viscosity-pressure-temperature data on the light hydrocarbons in their liquid, gas and dense-fluid regions, and to develop an equation which relates viscosity to the state properties. The form of the equation should be such that it will approach the kinetic-theory relationships for temperature dependence of viscosity for gases at low pressures, and for liquids at high densities. Background Most efforts it development of general relationships for prediction of fluid viscosities are based on equating the expression for momentum transfer per unit area, or shear force, developed by use of a molecular model to the defining equation for a continuous Newtonian fluid. JPT P. 210^