scholarly journals The apparent anticorrelation between the mass opacity of interstellar dust and the surface density of interstellar gas

2020 ◽  
Vol 494 (1) ◽  
pp. L48-L52 ◽  
Author(s):  
F D Priestley ◽  
A P Whitworth
Keyword(s):  
Spatial Variation ◽  
Surface Density ◽  
Interstellar Dust ◽  
Line Of Sight ◽  
Interstellar Gas ◽  
Dust Mass ◽  
Dust Evolution ◽  
Disc Galaxies

ABSTRACT Recent analyses of Herschel observations suggest that in nearby disc galaxies the dust mass opacity at $500 \, {\rm \mu m}$, κ500, decreases with increasing gas surface density, ΣISM. This apparent anticorrelation between κ500 and ΣISM is opposite to the behaviour expected from theoretical dust evolution models; in such models, dust in denser, cooler regions (i.e. regions of increased ΣISM) tends to grow and therefore to have increased κ500. We show, using a toy model, that the presence of a range of dust temperatures along the line of sight can lead to spuriously low estimated values of κ500. If in regions of higher ΣISM the range of dust temperatures extends to lower values (as seems likely), the magnitude of this effect may be sufficient to explain the apparent anticorrelation between κ500 and ΣISM. Therefore there may not be any need for spatial variation in the intrinsic dust properties that run counter to theoretical expectations.

scholarly journals Discussion following the Reports by Weaver and Field

1970 ◽  
Vol 39 ◽  
pp. 77-97
Author(s):  
B. F. Burke
Keyword(s):  
Electron Density ◽  
Gas Dynamics ◽  
Surface Density ◽  
Emission Measure ◽  
Indirect Evidence ◽  
Line Of Sight ◽  
Dispersion Measure ◽  
List Type ◽  
X Rays ◽  
Interstellar Gas

This long, very intensive, and partially very confused discussion, has been rearranged in six sections: (1) Direct and Indirect Evidence of X-rays and Low-Energy Cosmic Rays; (2) The Boundary Layer between the Stable Gas Phases; (3) Theoretical Aspects of Interstellar Gas Dynamics and the Formation of Clouds; (4) Observational Aspects of Interstellar Gas Dynamics and the Formation of Clouds; (5) Observations of the Rarefied, Neutral Intercloud Medium and of the Interstellar Electron Density; (6) The Dynamical Theory of Hii Regions. Section 2 has been transferred from the Discussion on Monday, September 8 (Chapter 2). To Section 6 have been added remarks made during various discussions. A couple of remarks have been transferred to other Discussions. Part of the Discussion (in Section 5) was very confused; an attempt has been made to condense and to make as much sense as possible out of what was said. For the convenience of the reader I recapitulate a few concepts, the (mis)-use of which lead partially to the confusion: (i)The hydrogen surface density or column density NH = ʃ nH dl (NH is sometimes called the hydrogen measure HM).(ii)The dispersion measure DM = ʃ ne dl (DM is often called the electron surface density Ne).(iii)The rotation measure RM = c1 ʃ neB‖ dl.(iv)The emission measure EM = ʃ ne2 dl.In these definitions, nH represents the hydrogen density, ne the electron density, B‖ the component of the magnetic field strength along the line of sight and l the distance along the line of sight. Conventionally nH and ne are expressed in cm−3, B in μG and l in pc. In addition there are two combinations of these quantities (and of the electron temperature T) involved in the Discussion:(v)The free-free absorption coefficient k(v) = c2(v)ne2T−3/2 (v is the frequency).(vi)The free-free emissivity ɛ(v)=c3(v)ne2T−1/2 (only at radio wavelengths). c2 and c3 depend also on T, but only rather weakly.

scholarly journals Systematic errors in dust mass determinations: insights from laboratory opacity measurements

2020 ◽  
Vol 499 (4) ◽  
pp. 4666-4686
Author(s):  
Lapo Fanciullo ◽  
Francisca Kemper ◽  
Peter Scicluna ◽  
Thavisha E Dharmawardena ◽  
Sundar Srinivasan
Keyword(s):  
Interstellar Dust ◽  
Thermal Emission ◽  
High Redshift ◽  
Synthetic Data ◽  
Far Infrared ◽  
Systematic Errors ◽  
Experimental Measurements ◽  
Dust Mass ◽  
Dusty Galaxies ◽  
Different Temperatures

ABSTRACT The thermal emission of dust is one of the most important tracers of the interstellar medium: multiwavelength photometry in the far-infrared (FIR) and submillimetre (submm) can be fitted with a model, providing estimates of the dust mass. The fit results depend on the assumed value for FIR/submm opacity, which in most models – due to the scarcity, until recently, of experimental measurements – is extrapolated from shorter wavelengths. Lab measurements of dust analogues, however, show that FIR opacities are usually higher than the values used in models and depend on temperature, which suggests that dust mass estimates may be biased. To test the extent of this bias, we create multiwavelength synthetic photometry for dusty galaxies at different temperatures and redshifts, using experimental results for FIR/submm dust opacity and then we fit the synthetic data using standard dust models. We find that the dust masses recovered by typical models are overestimated by a factor of 2–20, depending on how the experimental opacities are treated. If the experimental dust samples are accurate analogues of interstellar dust, therefore, current dust masses are overestimated by up to a factor of 20. The implications for our understanding of dust, both Galactic and at high redshift, are discussed.

scholarly journals High-velocity interstellar absorption associated with the supernova remnant W28

2020 ◽  
Vol 495 (3) ◽  
pp. 2909-2920 ◽  
Author(s):  
Adam M Ritchey
Keyword(s):  
High Velocity ◽  
Molecular Cloud ◽  
Supernova Remnant ◽  
Line Emission ◽  
Line Of Sight ◽  
Interstellar Gas ◽  
Lower Velocity ◽  
North East ◽  
The North ◽  
Visible Wavelengths

ABSTRACT We present an analysis of moderately high-resolution optical spectra obtained for the sightline to CD−23 13777, an O9 supergiant that probes high-velocity interstellar gas associated with the supernova remnant W28. Absorption components at both high positive and high negative velocity are seen in the interstellar Na i D and Ca ii H and K lines towards CD−23 13777. The high-velocity components exhibit low Na i/Ca ii ratios, suggesting efficient grain destruction by shock sputtering. High column densities of CH+, and high CH+/CH ratios, for the components seen at lower velocity may be indicative of enhanced turbulence in the clouds interacting with W28. The highest positive and negative velocities of the components seen in Na i and Ca ii absorption towards CD−23 13777 imply that the velocity of the blast wave associated with W28 is at least 150 km s−1, a value that is significantly higher than most previous estimates. The line of sight to CD−23 13777 passes very close to a well-known site of interaction between the supernova remnant and a molecular cloud to the north-east. The north-east molecular cloud exhibits broad molecular line emission, OH maser emission from numerous locations, and bright extended GeV and TeV γ-ray emission. The sightline to CD−23 13777 is thus a unique and valuable probe of the interaction between W28 and dense molecular gas in its environs. Future observations at ultraviolet and visible wavelengths will help to better constrain the abundances, kinematics, and physical conditions in the shocked and quiescent gas along this line of sight.

scholarly journals Observations of interstellar Lyman-α absorption

1970 ◽  
Vol 36 ◽  
pp. 281-301 ◽  
Author(s):  
Edward B. Jenkins
Keyword(s):  
Neutral Hydrogen ◽  
Local Region ◽  
Line Of Sight ◽  
Small Scale ◽  
Line Broadening ◽  
Interstellar Gas ◽  
B Stars ◽  
Hydrogen Density ◽  
The Galaxy ◽  
To Come

Absorption at the Lyman-α transition from interstellar neutral hydrogen has been observed in the ultraviolet spectra of 18 nearby O and B stars. Radiation damping is the dominant cause of line broadening, which makes the derived line-of-sight column densities proportional to the square of the observed equivalent widths. An average hydrogen density on the order of 0.1 atom cm−3 has been found for most of the stars observed so far. This is in contrast to the findings from surveys of 21-cm radio emission, which suggest 0.7 atom cm−3 exists in the local region of the Galaxy. Several effects which might introduce uncertainties into the Lyman-α measurements are considered, but none seems to be able to produce enough error to explain the disagreement with the 21-cm data. The possibility that small-scale irregularities in the interstellar gas could give significantly lower values at Lyman-α is explored. However, a quantitative treatment of the factor of ten discrepancy in Orion indicates the only reasonable explanation requires the 21-cm flux to come primarily from small, dense, hot clouds which are well separated from each other. The existence of such clouds, however, poses serious theoretical difficulties.

scholarly journals Formation of Molecular Hydrogen in Interstellar Space

1965 ◽  
Vol 7 ◽  
pp. 253-257
Author(s):  
H. F. P. Knaap ◽  
C. J. N. Van Den Meijdenberg ◽  
J. J. M. Beenakker ◽  
H. C. Van De Hulst
Keyword(s):  
Interstellar Dust ◽  
Interstellar Space ◽  
Interstellar Gas ◽  
Interstellar Clouds ◽  
Dark Clouds ◽  
Hydrogen Molecules ◽  
Large Factor ◽  
Dust Grain ◽  
Indirect Argument ◽  
Made In

Although Several Attempts at observing the interstellar hydrogen molecules in the ultraviolet or infrared are in preparation (ref. 1), these molecules are still undetected. They may form the most abundant unobserved constituent of the interstellar gas. The strongest indirect argument for the presence of these molecules lies in the fact that the density of atomic hydrogen observed by the 21-cm line goes down in some dark clouds, where the dust density and, presumably, the total gas density goes up by a large factor.Inasmuch as the density in the interstellar clouds is of the order of 10 atoms/cm3 and the temperature is only of the order of 100° K, any formation of molecules by atom-atom collisions is too slow to be of importance. The most eligible process for H2 formation is recombination on the surface of an interstellar dust grain. Rate estimates of this process have been made in various degrees of detail, as reported in references 2 to 4.

A Discussion on the physics of the solar atmosphere - Spectroscopic studies of the solar corona at x-ray wavelengths

1976 ◽  
Vol 281 (1304) ◽  
pp. 383-390 ◽  
Keyword(s):  
Spatial Distribution ◽  
Solar Corona ◽  
Spatial Variation ◽  
Resonance Line ◽  
Active Regions ◽  
Resonance Scattering ◽  
Spectroscopic Studies ◽  
Line Of Sight ◽  
X Ray ◽  
Forbidden Transitions

The spatial distribution of the emission in several X-ray lines is discussed with emphasis on temperature dependence and association with active regions. New results are presented for the trio of helium-like O vii lines which demonstrate (1) a spatial variation in the density dependent forbidden to intersystem line ratio, and (2) a strong spatial variation in the intensity of the O vii resonance line relative to the optically forbidden transitions. The second effect appears to be caused by resonance scattering by material in the line of sight.

scholarly journals Astrochemical relevance of VUV ionization of large PAH cations

2020 ◽  
Vol 641 ◽  
pp. A98 ◽  
Author(s):  
G. Wenzel ◽  
C. Joblin ◽  
A. Giuliani ◽  
S. Rodriguez Castillo ◽  
G. Mulas ◽  
...  
Keyword(s):  
Cross Sections ◽  
Density Functional ◽  
Interstellar Dust ◽  
Vacuum Ultraviolet ◽  
Branching Ratio ◽  
Molecular Size ◽  
Photoelectric Effect ◽  
Interstellar Gas ◽  
Ionization Cross Sections ◽  
The Impact

Context. As part of interstellar dust, polycyclic aromatic hydrocarbons (PAHs) are processed by an interaction with vacuum ultraviolet (VUV) photons emitted by hot stars. This interaction leads to the emission of not only the well-known aromatic infrared bands, but also electrons, which can significantly contribute to the heating of the interstellar gas. Aims. Our aim is to investigate the impact of molecular size on the photoionization properties of cationic PAHs. Methods. Trapped PAH cations of sizes between 30 and 48 carbon atoms were submitted to VUV photons in the range of 9–20 eV from the DESIRS beamline at the synchrotron SOLEIL. All resulting photoproducts including dications and fragment cations were mass-analyzed and recorded as a function of photon energy. Results. Photoionization is found to be predominant over dissociation at all energies, which differs from the conclusions of an earlier study on smaller PAHs. The photoionization branching ratio reaches 0.98 at 20 eV for the largest studied PAH. The photoionization threshold is observed to be between 9.1 and 10.2 eV, in agreement with the evolution of the ionization potential with size. Ionization cross sections were indirectly obtained and photoionization yields extracted from their ratio with theoretical photoabsorption cross sections, which were calculated using time-dependent density functional theory. An analytical function was derived to calculate this yield for a given molecular size. Conclusions. Large PAH cations could be efficiently ionized in H I regions and contribute to the heating of the gas by the photoelectric effect. Also, at the border of or in H II regions, PAHs could be exposed to photons of energy higher than 13.6 eV. Our work provides recipes to be used in astronomical models to quantify these points.

scholarly journals The role of wind driving in OB star bow nebulae

2020 ◽  
Vol 494 (2) ◽  
pp. 1838-1847
Author(s):  
Curtis Struck
Keyword(s):  
Interstellar Dust ◽  
Gas Flow ◽  
Ionization Front ◽  
Emission Region ◽  
Interstellar Gas ◽  
Weibel Instability ◽  
Peculiar Velocities ◽  
Interstellar Dust Grains ◽  
Energy And Momentum ◽  
Ir Emission

ABSTRACT Bow-shaped mid-infrared (mid-IR) emission regions have been discovered in satellite observations of numerous late-type O and early-type B stars with moderate velocities relative to the ambient interstellar medium. Previously, hydrodynamical bow shock models have been used to study this emission. It appears that such models are incomplete in that they neglect kinetic effects associated with long mean free paths of stellar wind particles, and the complexity of Weibel instability fronts. Wind ions are scattered in the Weibel instability and mix with the interstellar gas. However, they do not lose their momentum and most ultimately diffuse further into the ambient gas like cosmic rays, and share their energy and momentum. Lacking other coolants, the heated gas transfers energy into interstellar dust grains, which radiate it. This process, in addition to grain photoheating, provides the energy for the emission. A weak R-type ionization front, formed well outside the IR emission region, generally moderates the interstellar gas flow into the emission region. The theory suggests that the IR emission process is limited to cases of moderate stellar peculiar velocities, evidently in accord with the observations.

scholarly journals Interstellar Dust and the H2 Molecule

1965 ◽  
Vol 7 ◽  
pp. 259-264
Author(s):  
K. H. Schmidt
Keyword(s):  
Hydrogen Atom ◽  
Molecular Hydrogen ◽  
Interstellar Dust ◽  
Unit Volume ◽  
Formation Rate ◽  
Dust Particles ◽  
Interstellar Gas ◽  
The Past ◽  
Grain Surface ◽  
H2 Molecule

Nearly 20 Years Ago van de Hulst stated that the formation of molecular hydrogen occurs on the surfaces of the interstellar grains. (See ref. 1.) In the last years several authors discussed the problem of the interstellar abundance of the H2 molecule. (See refs. 2 to 9.) They all found that the percentage of the molecular hydrogen in the interstellar gas probably is much larger than had been thought in the past and that the essential mechanism of H2 formation is the formation on the particle surfaces. Therefore, the formation rate of interstellar H2 is a function of the area of the grain surface per unit volume, which is dependent on the average radius of the grains ā, on the number of dust particles per unit volume N(ā), and on the distribution function of the particle radii. The formation rate is determined by the density of the atomic hydrogen nH and the temperature of the interstellar gas Tgas. Finally, the formation rate of H2 depends on the probability π that an impinging hydrogen atom on a grain joins with another hydrogen atom to form a molecule.

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