On the Ionization of Warm Opaque Interstellar Clouds and the Intercloud Medium

1997 ◽  
Vol 488 (1) ◽  
pp. 234-240 ◽  
Author(s):  
D. W. Sciama
Keyword(s):  
Interstellar Clouds ◽  
Intercloud Medium

scholarly journals Abundances in Diffuse Interstellar Clouds

1980 ◽  
Vol 5 ◽  
pp. 293-300 ◽  
Author(s):  
M. Jura
Keyword(s):  
Interstellar Extinction ◽  
Future Research ◽  
Chemical Compositions ◽  
Interstellar Clouds ◽  
Dark Clouds ◽  
Neutral Gas ◽  
Self Gravity ◽  
Ultraviolet Observations ◽  
Diffuse Clouds ◽  
Intercloud Medium

Interstellar clouds are concentrations of cold (T ≲ 100 K) neutral gas (cf. Spitzer 1978) which are immersed within an intercloud medium. It is worthwhile to distinguish between diffuse clouds (roughly those with E[B-V] ≳ 0.5) and dark clouds (those with E[B-V] ≳ 0.5). This distinction is useful in the sense that diffuse clouds are relatively warm (T ∼ 100 K), they are composed mostly of atomic species except for hydrogen which can be appreciably molecular, and they are dynamically controlled by their interaction with the intercloud medium. Dark clouds are relatively cold (T ∼ 10 K), they contain a rich variety of molecules, and self-gravity is important in their evolution. Because the interstellar extinction is a rapid function of wavelength, most ultraviolet observations have been of diffuse clouds. The IUE satellite is sufficiently powerful that observations of some dark clouds are possible, and an important area of future research will be to delineate more quantitatively the similarities and differences between diffuse clouds and dark clouds.With ultraviolet observations, considerable progress has been made in understanding the physical characteristics of clouds including determinations of their densities, temperatures, chemical compositions and dynamics (cf. Spitzer and Jenkins 1976). Because particular progress has been made on understanding the abundances within diffuse clouds and because of the limitations of space, we restrict this review to a discussion of abundances within diffuse clouds. These abundance measurements provide a set of fundamental astrophysical data.

scholarly journals The Gas-to-Dust Ratio in the Galaxy

1974 ◽  
Vol 60 ◽  
pp. 179-182
Author(s):  
F. J. Kerr ◽  
G. R. Knapp
Keyword(s):  
Globular Clusters ◽  
Galactic Latitude ◽  
Interstellar Clouds ◽  
The Galaxy ◽  
Intercloud Medium

We have investigated the gas-to-dust ratio in the Galaxy by comparing 21-cm Hicolumn densities with the color excesses of globular clusters. We find a constant gas-to-reddening ratio in interstellar clouds and the intercloud medium. This ratio is also independent of galactic latitude.

scholarly journals Low- and Intermediate-Velocity Hydrogen at Positive Galactic Latitudes

1974 ◽  
Vol 60 ◽  
pp. 631-635
Author(s):  
K. Takakubo
Keyword(s):  
Low Velocity ◽  
Interstellar Clouds ◽  
Hydrogen Emission ◽  
Intercloud Medium

Low- and intermediate-velocity Gaussian components of hydrogen emission profiles observed at positive galactic latitudes are investigated. The low-velocity hole, as called by Wesselius and Fejes (1973), is a phenomenon which appears in the layer composed of the gas emitting narrow Gaussian components but does not in that of wide components. Both Models I and II suggested by Wesselius and Fejes contradict this result. A hypothesis is proposed that a stream motion in the layer emitting wide Gaussian components, which may be identified with the intercloud medium, swept away the interstellar clouds emitting narrow Gaussian components in the region of the hole.

scholarly journals A Modern Look at ‘Interstellar Clouds’

1974 ◽  
Vol 60 ◽  
pp. 13-44
Author(s):  
Carl Heiles
Keyword(s):  
Magnetic Field ◽  
Cloud Model ◽  
Scale Structure ◽  
Doppler Velocity ◽  
Small Scale ◽  
Interstellar Gas ◽  
Interstellar Clouds ◽  
Small Scale Structure ◽  
Oriented Parallel ◽  
Intercloud Medium

We compare past and present modes of investigation of the structure of the interstellar gas. Many aspects of the interstellar cloud model are invalid.Interstellar optical absorption lines and Hi21-cm emission lines show a number of very large aggregates with properties similar to those of ‘cloud complexes’. At nonzero velocities especially for b<0°, exist optical lines which have no Hicounterparts. These are almost certainly produced in low-density gas clouds; perhaps the intercloud medium is itself cloudy.Maps of Hicolumn density taken over large velocity ranges do not reveal much small-scale structure. This fact cannot easily be reconciled with the statistical analyses of interstellar reddening. The maps do reveal large, coherent gas structures which are often filamentary in shape and at least sometimes aligned parallel to the interstellar magnetic field.Maps of Hicolumn density over small velocity ranges show much small-scale structure, often filamentary in shape. The filaments are almost universally oriented parallel to the interstellar magnetic field and have Doppler velocity gradients along their lengths. In one area the geometry of the field and gas almost exclusively suggests Alfvén-type motions.

scholarly journals Hot Gas in the Galaxy: How Extensive is it?

1978 ◽  
Vol 77 ◽  
pp. 139-146
Author(s):  
Frank H. Shu
Keyword(s):  
Physical State ◽  
Substantial Fraction ◽  
Ionized Gas ◽  
Interstellar Clouds ◽  
X Ray ◽  
Hot Gas ◽  
The Galaxy ◽  
Intercloud Medium

Recent satellite detections of 0 VI absorption and soft X-ray emission leave little doubt that, locally, a substantial fraction of the volume of space between interstellar clouds must be filled with rarified and highly ionized gas at temperatures ranging from 2 × 105 to > 106 K. (See the reviews of Spitzer and Jenkins 1975, and Kraushaar 1977.) The physical state of this gas contrasts sharply with the theoretical picture of a largely neutral, warm, intercloud medium at ∼ 104 K developed by Pikel'ner (1967), Field, Goldsmith, and Habing (1969), and Spitzer and Scott (1969). My purpose here is to review the evidence, observational and theoretical, concerning how extensive hot gas at ∼ 106 K might be.

Dynamics of comets in the collapsing protosolar nebula

1999 ◽  
Vol 173 ◽  
pp. 45-50
Author(s):  
L. Neslušan
Keyword(s):  
Solar System ◽  
Velocity Distribution ◽  
Oort Cloud ◽  
Interstellar Clouds ◽  
Protosolar Nebula ◽  
Moving Bodies

AbstractComets are created in the cool, dense regions of interstellar clouds. These macroscopic bodies take place in the collapse of protostar cloud as mechanically moving bodies in contrast to the gas and miscroscopic dust holding the laws of hydrodynamics. In the presented contribution, there is given an evidence concerning the Solar system comets: if the velocity distribution of comets before the collapse was similar to that in the Oort cloud at the present, then the comets remained at large cloud-centric distances. Hence, the comets in the solar Oort cloud represent a relict of the nebular stage of the Solar system.

Modelling polarized radiation from interstellar clouds

2008 ◽  
Vol 31 ◽  
pp. 179-180
Author(s):  
M. Juvela ◽  
J. Goncalves ◽  
V.-M. Pelkonen ◽  
T. Lunttila
Keyword(s):  
Interstellar Clouds ◽  
Polarized Radiation

Preliminary Observations of Interstellar Sodium D-Line Absorption towards the Carina Nebula

1980 ◽  
Vol 4 (1) ◽  
pp. 95-97 ◽  
Author(s):  
J. B. Whiteoak ◽  
F. F. Gardner
Keyword(s):  
High Resolution ◽  
Hii Regions ◽  
Early Type ◽  
Interstellar Clouds ◽  
Carina Nebula ◽  
Line Absorption ◽  
General Investigation ◽  
Early Type Stars ◽  
Resolution Study

As part of a general investigation of interstellar clouds associated with southern HII regions we have begun a high-resolution study of the sodium D-line absorption in the directions of early-type stars that are likely to be associated with or located behind the clouds.

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