scholarly journals Rotationally Excited H2 in the Magellanic Clouds

2012 ◽  
Vol 8 (S292) ◽  
pp. 255-255
Author(s):  
Rui Xue ◽  
Daniel Welty ◽  
Tony Wong
Keyword(s):  
Column Density ◽  
Velocity Dispersion ◽  
Volume Density ◽  
Magellanic Clouds ◽  
Survey Study ◽  
Standard Line ◽  
Formation Conditions ◽  
Dust Modeling ◽  
Gas Volume ◽  
Comprehensive Study

AbstractWe have performed a survey study of rotational excited-state H2 Lyman-Werner absorption lines in the entire FUSE Magellanic Clouds Legacy archive. These lines reflect the UV pumping and formation conditions of H2, enabling a more comprehensive study of H2 gas properties, e.g. J-level populations N(J) and b-values (generally indicating the velocity dispersion). Combining with our previous measurements of N(Hi) and N(H2), we derived H2 excitation temperatures, gas volume density n(H), and local UV radiation field strength IUV for each sight line. The results indicate a weaker correlation between n(H) and IUV in Magellanic Clouds than the Galactic sight lines. We also obtained N(H)/E(B − V) ratios from the Spitzer-SAGE and previous CO J = 1 − 0 / Hi 21 cm surveys at sight line locations, using dust modeling and standard line brightness-column density conversion factors. They show a roughly linear correlation with absorption-based N(H)/E(B − V) values, and have a similar scatter (∼0.7 dex) across the LMC and SMC.

scholarly journals Dynamical History of the Solar Neighbourhood

1980 ◽  
Vol 85 ◽  
pp. 191-193
Author(s):  
J. P. Vader
Keyword(s):  
Velocity Dispersion ◽  
Galactic Plane ◽  
Accretion Rate ◽  
Mass Density ◽  
Dynamical Evolution ◽  
Volume Density ◽  
Solar Neighbourhood ◽  
Stellar Velocity ◽  
Gas Accretion ◽  
Gas Volume

The dynamical evolution of the solar neighbourhood is described by an accretion model in which the gas accretion rate decays exponentially with time. Stars form at a rate proportional to the local gas volume density and their velocity dispersion is increased after birth by star-cloud collisions. The present mass density distribution of stars and of gas perpendicular to the galactic plane (Oort 1965) and the observed increase of stellar velocity dispersion with age (Mayor 1974; Mayor and Martinet 1977) are reproduced for an e-folding time of 3 × 109 y of the gas accretion rate and a characteristic star formation time scale of 2.8 × 109 y.

scholarly journals Magnetized filament models for diverging plasma lenses

2020 ◽  
Vol 493 (2) ◽  
pp. 1736-1752
Author(s):  
Adam Rogers ◽  
Abdul Mohamed ◽  
Bailey Preston ◽  
Jason D Fiege ◽  
Xinzhong Er
Keyword(s):  
Magnetic Field ◽  
Column Density ◽  
Volume Density ◽  
Magnetized Plasma ◽  
Gravitational Lens ◽  
Line Of Sight ◽  
Density Distributions ◽  
Rotation Measure ◽  
Spherical Plasma ◽  
Helical Magnetic Field

ABSTRACT Spherical plasma lens models are known to suffer from a severe overpressure problem, with some observations requiring lenses with central pressures up to millions of times in excess of the ambient interstellar medium. There are two ways that lens models can solve the overpressure problem: a confinement mechanism exists to counter the internal pressure of the lens, or the lens has a unique geometry, such that the projected column-density appears large to an observer. This occurs with highly asymmetric models, such as edge-on sheets or filaments, with potentially low volume–density. In the first part of this work we investigate the ability of non-magnetized plasma filaments to mimic the magnification of sources seen behind spherical lenses and we extend a theorem from gravitational lens studies regarding this model degeneracy. We find that for plasma lenses, the theorem produces unphysical charge density distributions. In the second part of the work, we consider the plasma lens overpressure problem. Using magnetohydrodynamics, we develop a non self-gravitating model filament confined by a helical magnetic field. We use toy models in the force-free limit to illustrate novel lensing properties. Generally, magnetized filaments may act as lenses in any orientation with respect to the observer, with the most high-density events produced from filaments with axes near the line of sight. We focus on filaments that are perpendicular to the line of sight that show the toroidal magnetic field component may be observed via the lens rotation measure.

scholarly journals The relation between the turbulent Mach number and observed fractal dimensions of turbulent clouds

2019 ◽  
Vol 488 (2) ◽  
pp. 2493-2502 ◽  
Author(s):  
James R Beattie ◽  
Christoph Federrath ◽  
Ralf S Klessen ◽  
Nicola Schneider
Keyword(s):  
Fractal Dimension ◽  
Star Formation ◽  
Mach Number ◽  
Column Density ◽  
Velocity Dispersion ◽  
Error Function ◽  
Empirical Relation ◽  
Three Dimensional ◽  
Fractal Dimensions ◽  
Cloud Kinematics

Abstract Supersonic turbulence is a key player in controlling the structure and star formation potential of molecular clouds (MCs). The three-dimensional (3D) turbulent Mach number, $\operatorname{\mathcal {M}}$, allows us to predict the rate of star formation. However, determining Mach numbers in observations is challenging because it requires accurate measurements of the velocity dispersion. Moreover, observations are limited to two-dimensional (2D) projections of the MCs and velocity information can usually only be obtained for the line-of-sight component. Here we present a new method that allows us to estimate $\operatorname{\mathcal {M}}$ from the 2D column density, Σ, by analysing the fractal dimension, $\mathcal {D}$. We do this by computing $\mathcal {D}$ for six simulations, ranging between 1 and 100 in $\operatorname{\mathcal {M}}$. From this data we are able to construct an empirical relation, $\log \operatorname{\mathcal {M}}(\mathcal {D}) = \xi _1(\operatorname{erfc}^{-1} [(\mathcal {D}-\operatorname{\mathcal {D}_\text{min}})/\Omega ] + \xi _2),$ where $\operatorname{erfc}^{-1}$ is the inverse complimentary error function, $\operatorname{\mathcal {D}_\text{min}}= 1.55 \pm 0.13$ is the minimum fractal dimension of Σ, Ω = 0.22 ± 0.07, ξ1 = 0.9 ± 0.1, and ξ2 = 0.2 ± 0.2. We test the accuracy of this new relation on column density maps from Herschel observations of two quiescent subregions in the Polaris Flare MC, ‘saxophone’ and ‘quiet’. We measure $\operatorname{\mathcal {M}}\sim 10$ and $\operatorname{\mathcal {M}}\sim 2$ for the subregions, respectively, which are similar to previous estimates based on measuring the velocity dispersion from molecular line data. These results show that this new empirical relation can provide useful estimates of the cloud kinematics, solely based upon the geometry from the column density of the cloud.

scholarly journals Clouds, cores, and stars in the nearest molecular complexes

1991 ◽  
Vol 147 ◽  
pp. 221-228
Author(s):  
P. C. Myers
Keyword(s):  
Column Density ◽  
Velocity Dispersion ◽  
Molecular Complexes ◽  
Star Cluster ◽  
The Sun ◽  
Core Size ◽  
Core Mass ◽  
The Core ◽  
Internal Motions ◽  
Over Time

The properties and structure of six molecular complexes within 500 pc of the Sun are described and compared. They are generally organized into elongated filaments which appear connected to less elongated, more massive clouds. Their prominent star clusters tend to be located in the massive clouds rather than in the filaments. The complexes have similar structure, but big differences in scale, from a few pc to some 30 pc. They show a pattern of regional virial equilibrium, where the massive, centrally located clouds are close to virial equilibrium, while the less massive filaments and other small clouds have too little mass to bind their observed internal motions. Complexes can be ranked according to increasing size, mass, core mass, and the mass and number of the associated stars: they range from Lupus to Taurus to Ophiuchus to Perseus to Orion B to Orion A. The cores in nearby complexes tend to have maps which are elongated, rather than round. The core size, velocity dispersion, and column density of most cores are consistent with virial equilibrium. Cores in Orion tend to exceed cores in Taurus in their line width, size, temperature, mass, and in the mass of the associated star, if any. Stars in Orion tend to be more numerous and more massive than in Taurus, while those in Taurus tend to be more numerous and more massive than in Lupus. The mass of a core tends to increase with the mass of the cloud where it is found, with the mass of the star cluster with which it is associated, and with its proximity to a star cluster. These properties suggest that complexes and their constituent cores and clusters develop together over time, perhaps according to the depth of the gravitational well of the complex.

scholarly journals Neutral Hydrogen in the Magellanic Clouds

1999 ◽  
Vol 190 ◽  
pp. 37-44
Author(s):  
L. Staveley-Smith ◽  
S. Kim ◽  
S. Stanimirović
Keyword(s):  
Atomic Hydrogen ◽  
Large Scale ◽  
Milky Way ◽  
Velocity Dispersion ◽  
Neutral Hydrogen ◽  
Distribution Functions ◽  
Magellanic Clouds ◽  
Self Gravity ◽  
Rotational Shear ◽  
Compact Array

We review observations of neutral atomic hydrogen (HI) in the Magellanic Clouds (MCs). Being the nearest gas-rich neighbours of the Milky Way the MCs give us an excellent opportunity to study in detail the structure and evolution of the interstellar medium (ISM) and the effect of interactions between galaxies. HI in emission provides a probe of the structure and velocity field of the Clouds, allowing the study of their velocity dispersion, 3-D structure, and large-scale total-mass distribution. Recent data from Australia Telescope Compact Array surveys reveal a morphology (for both Clouds) which is heavily dominated by the effects of local star-formation, rotational shear, fragmentation, self-gravity and turbulence. The new data, which has a spatial resolution down to 10 pc, also allows the study of the distribution functions in velocity and mass for HI clouds. We discuss the morphology, dynamics and giant shell population of the LMC and SMC.

scholarly journals Mass-to-light ratio of intermediate and old clusters in the Magellanic Clouds

1991 ◽  
Vol 148 ◽  
pp. 213-214
Author(s):  
Patrick Seitzer
Keyword(s):  
Velocity Dispersion ◽  
Stellar System ◽  
Velocity Distribution Function ◽  
Magellanic Clouds ◽  
Core Radius ◽  
Echelle Spectrograph ◽  
The Past ◽  
Multiple Observations ◽  
Ccd Detector ◽  
Galactic Clusters

The observed central velocity dispersion of a stellar system is dependent on the mass distribution and the velocity distribution function. For the past several years a program has been undertaken to determine central velocity dispersions of LMC and SMC clusters for comparison with galactic clusters. The mass-to-light ratio (M/L) estimates so obtained should be considerably more accurate than those obtained by tidal arguments (Chun 1978, Elson & Freeman 1985).Accurate radial velocities (mean error = 1.3 km s−1) have been obtained for a sample of K-type giants with the CTIO 4m telescope and échelle spectrograph with CCD detector. All of the stars observed were within two core radii of the cluster centre. Multiple observations at several epochs (1984 and 1989) were used to determine the velocity errors.The central M/L in solar units has been estimated from single component isotropic King models (King 1966), with the assumption that the distribution of mass follows that of the light. Observed values for the total integrated light and core radius were taken from the literature (in particular, from Chun 1978, van den Bergh 1981, Mateo 1987).

Xvoigt: An Interactive Absorption Line Fitting Program for the X Window System

1995 ◽  
Vol 12 (2) ◽  
pp. 239-243 ◽  
Author(s):  
David P. Max ◽  
Geoff Bailey
Keyword(s):  
Absorption Line ◽  
Column Density ◽  
Velocity Dispersion ◽  
Absorption Lines ◽  
Signal To Noise ◽  
Window System ◽  
Profile Fitting ◽  
X Window ◽  
Line Fitting ◽  
Line Analysis

AbstractWe have developed an easy-to-use, mouse-driven program for the interactive fitting of interstellar absorption lines in high-resolution astronomical spectra. The program, Xvoigt, gives values for the column density and velocity dispersion of the absorbing clouds. It runs under the popular X Window system available on most workstations, and offers significant enhancements over existing profile-fitting software. Xvoigt can be an important adjunct to automatic programs for fitting absorption lines in low to moderate signal-to-noise QSO or other spectra, and is ideal for demonstrating the details and difficulties of absorption line analysis.

scholarly journals Ultra-Rapid Uptake and Highly Stable Storage of Methane as Combustible Ice

2020 ◽  
Author(s):  
Marcus N. Goh ◽  
Gaurav Bhattacharjee ◽  
Sonia E.K. Arumuganainar ◽  
Praveen Linga
Keyword(s):  
Natural Gas ◽  
Large Scale ◽  
Storage System ◽  
Formation Rate ◽  
Hydrate Formation ◽  
High Volume ◽  
Volume Density ◽  
Long Term Storage ◽  
Dual Action ◽  
Gas Volume

<p>Ever-increasing natural gas (NG) consumption trends due to its cleanest tag and abundant availability point towards an inevitable transition into an NG dominated economy. Solidified Natural Gas (SNG) storage via combustible ice or clathrate hydrates presents an economically sound prospect, promising high volume density, and long-term storage. Here we establish 1,3-dioxolane (DIOX), as a highly efficient dual-action (thermodynamic and kinetic promoter) additive for clathrate (methane sII) hydrate formation. By synergistically combining a small concentration (300 ppm) of kinetic promoter L-tryptophan with DIOX, we further demonstrate ultra-rapid hydrate formation with a methane uptake of 83.81 (±0.77) volume of gas/volume of hydrate (v/v) within 15 minutes. To the best of our knowledge, this is the fastest reaction time ever reported for sII hydrates related to SNG technology and represents a 147% increase in the hydrate formation rate compared to the standard water-DIOX system. Mixed methane-DIOX hydrates in pelletized form also exhibit incredible stability when stored at atmospheric pressure and moderate temperature of 268.15 K, thereby showcasing potential to be industrially adoptable for the development of a large-scale NG storage system.</p>

scholarly journals The Galactic HI Halo

1997 ◽  
Vol 166 ◽  
pp. 479-482
Author(s):  
P.M.W. Kalberla ◽  
G. Westphalen ◽  
U. Mebold ◽  
D. Hartmann ◽  
W.B. Burton
Keyword(s):  
Emission Spectrum ◽  
Radiation Effects ◽  
Column Density ◽  
Velocity Dispersion ◽  
Scale Height ◽  
Hydrostatic Equilibrium ◽  
High Dispersion ◽  
Model Parameters ◽  
Stray Radiation

AbstractWe find indications for diffuse HI gas at substantial z heights in our Galaxy, with a velocity dispersion of 60 km s−1 and a vertical projected column density of 1.41019 cm−2. This pervasive component of the emission spectrum could be identified in the Leiden/Dwingeloo 21 cm Survey (LDS) after increasing the accuracy further by correcting the observations for reflections from ground. Assuming hydrostatic equilibrium an exponential scale height of 4.4 kpc for the observed diffuse high-dispersion Hi component is deduced. This differs from the scale height of lkpc derived by Lockman & Gehman (1991), which corresponds to a velocity dispersion of 34kms−1, based on an analysis of the the Bell Laboratories HI Survey (BLS). A comparison of BLS and LDS data explains the differences in the derived model parameters in terms of baseline uncertainties at a level of ≈ 30 mK. We find additional indications for baseline uncertainties in the BLS data. Concerning the LDS we cannot, however, exclude that this survey may also be affected by baseline uncertainties. Receiver bandpass and stray-radiation effects need a more thorough analysis before drawing firm conclusions.

scholarly journals Ammonia clumps in the Orion and Cepheus clouds

1991 ◽  
Vol 147 ◽  
pp. 436-437
Author(s):  
J. Harju ◽  
C.M. Walmsley ◽  
J.G.A. Wouterloot
Keyword(s):  
Density Distribution ◽  
Column Density ◽  
Velocity Dispersion ◽  
Line Emission ◽  
Main Interest ◽  
Molecular Line ◽  
Dynamical State

We present statistics of clump properties in the Orion and Cepheus cloud complexes based on ammonia mapping observations. Surroundings of about 50 IRAS sources earlier found to have associated molecular line emission (Wouterloot, Walmsley and Henkel, 1988) were mapped in NH3(1,1) and (2,2) with the Effelsberg 100-m telescope. Our main interest has been in determining the clump sizes and masses on the basis of the ammonia column density distribution, which together with the observed velocity dispersion lead to a rough estimate of the dynamical state. We also have studied the star-clump separations which should give us estimates of the source ages. Special attention has been paid to comparison of our Orion data with the Benson and Myers (1989, hereafter BM89) results in Taurus because the linear resolutions in the two studies are similar.

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