Abundances in Diffuse Interstellar Clouds

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.

Download Full-text

Optical and ultraviolet observations of diffuse interstellar clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900009578 ◽

1997 ◽

Vol 178 ◽

pp. 407-420 ◽

Cited By ~ 2

Author(s):

David M. Meyer

Keyword(s):

High Resolution ◽

Velocity Structure ◽

Hubble Space Telescope ◽

Binary Star ◽

Small Scale ◽

Vibrationally Excited ◽

Interstellar Clouds ◽

Molecular Abundances ◽

Ultraviolet Observations ◽

Diffuse Clouds

Absorption-line studies of diffuse interstellar clouds have recently been invigorated at ultraviolet wavelengths with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope and at optical wavelengths with efficient high resolution spectrographs and CCD detectors. These instruments have made it possible to explore new regimes in determining the atomic and molecular abundances of diffuse clouds and tracing the velocity structure of interstellar lines. In the case of the atomic gas, the abundances of elements as diverse as oxygen and krypton have been accurately measured with GHRS and are consistent with a local ISM whose metallicity is about 2/3 that of the Sun. GHRS has also provided new insight on molecular processes in diffuse clouds through observations of CO, C2, HCl, and vibrationally-excited H2. With velocity resolutions of 0.3 km s−1 now attainable, optical spectra of species like CH, CH+, and CN are beginning to probe the physical characteristics of diffuse clouds in detail. Similar spectra of interstellar Na I toward resolvable binary star systems have recently revealed a rich variety of small-scale structure in the cold diffuse gas.

Download Full-text

Optical Observations of Nearby Interstellar Gas

International Astronomical Union Colloquium ◽

10.1017/s0252921100098080 ◽

1984 ◽

Vol 81 ◽

pp. 113-116 ◽

Cited By ~ 1

Author(s):

P.C. Frisch ◽

D.G. York

Keyword(s):

Optical Observations ◽

Low Density ◽

Interstellar Gas ◽

Interstellar Clouds ◽

Ratio Limit ◽

Ultraviolet Observations ◽

Diffuse Clouds ◽

Absorption Features ◽

Neutral Sodium ◽

Cloud Material

The yellow D lines from neutral sodium are some of the most useful interstellar absorption features for tracing out low density optically thin interstellar material. Hobbs (1978) compared column densities of Nal as determined from the D lines with HI column densities derived from ultraviolet observations and found values of N(NaI)/N(H)<6.3x10-9 in 18 sight-lines with very low reddening, with the exact value of the ratio apparently a function of temperature. If this ratio limit holds for diffuse clouds in nearby space, we expect that observations of the Nal D2 lines at the 3 mA level can be used to trace out nearby clouds with column densities in the range of ≥2.5x1018 cm-2. Thus sensitive measurements of the Nal D lines are a useful way of observing nearby neutral cloud material. With this in mind, we have chosen high-resolution observations of the yellow sodium lines as a means of locating and mapping nearby low density interstellar clouds.

Download Full-text

The Transition from Diffuse to Dense Clouds

Australian Journal of Physics ◽

10.1071/ph920543 ◽

1992 ◽

Vol 45 (4) ◽

pp. 543 ◽

Cited By ~ 12

Author(s):

Theodore P Snow

Keyword(s):

Gas Phase ◽

Open Cluster ◽

Dust Particles ◽

Interstellar Clouds ◽

Dark Clouds ◽

Diffuse Interstellar Bands ◽

Cloud Characteristics ◽

Wide Range ◽

Diffuse Clouds ◽

The Impact

Optical and ultraviolet spectroscopy have shown that diffuse interstellar clouds can have a wide range of properties, with especially large variations in the nature of the UV extinction curve and the abundances of molecular species. More subtle variations are found in the properties of the diffuse interstellar bands, and there have been suggestions that elemental depletions from the gas phase into solid dust particles also vary significantly. It is the purpose of this paper to review studies of the relatively diffuse interstellar clouds where these variations occur, and to explore the possible relationship between dust properties, as indicated by UV extinction, and other cloud characteristics. The focus is on relatively dense diffuse clouds, which may be viewed as transitional or intermediate between ordinary diffuse clouds and dark clouds, because in principle the greatest amount of information is available for the intermediate clouds, and because they serve as indicators of processes that may occur in the denser molecular clouds. The paper begins with a brief review of some results from the literature on transitional or intermediate clouds, and then provides a summary of some recent results on one particular cloud, in front of the star BD+31 �643, in the small open cluster IC348, which is part of the Perseus II complex of dark clouds and OB associations. The paper concludes with some tentative speculations about the possible status of the transitional clouds, along with a brief mention of the impact of upcoming instrumental developments on research in this area.

Download Full-text

Insights into the physics when modelling cold gas clouds in a hot plasma

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slz144 ◽

2019 ◽

Vol 490 (1) ◽

pp. L52-L56

Author(s):

Bastian Sander ◽

Gerhard Hensler

Keyword(s):

Adaptive Mesh Refinement ◽

Adaptive Mesh ◽

Careful Consideration ◽

Necessary Condition ◽

Interstellar Clouds ◽

Hot Plasma ◽

Magnetic Dipole Field ◽

Set Up ◽

Self Gravity ◽

Cold Gas

ABSTRACT This paper aims at studying the reliability of a few frequently raised, but not proven, arguments for the modelling of cold gas clouds embedded in or moving through a hot plasma and at sensitizing modellers to a more careful consideration of unavoidable acting physical processes and their relevance. At first, by numerical simulations we demonstrate the growing effect of self-gravity on interstellar clouds and, by this, moreover argue against their initial set-up as homogeneous. We apply the adaptive-mesh refinement code flash with extensions to metal-dependent radiative cooling and external heating of the gas, self-gravity, mass diffusion, and semi-analytic dissociation of molecules, and ionization of atoms. We show that the criterion of Jeans mass or Bonnor–Ebert mass, respectively, provides only a sufficient but not a necessary condition for self-gravity to be effective, because even low-mass clouds are affected on reasonable dynamical time-scales. The second part of this paper is dedicated to analytically study the reduction of heat conduction by a magnetic dipole field. We demonstrate that in this configuration, the effective heat flow, i.e. integrated over the cloud surface, is suppressed by only 32 per cent by magnetic fields in energy equipartition and still insignificantly for even higher field strengths.

Download Full-text

The Mass Spectrum of Interstellar Clouds

International Astronomical Union Colloquium ◽

10.1017/s0252921100024325 ◽

1989 ◽

Vol 120 ◽

pp. 511-517

Author(s):

John M. Dickey ◽

R. W. Garwood

Keyword(s):

Cross Sections ◽

Molecular Clouds ◽

Column Density ◽

Unit Volume ◽

Empirical Relationship ◽

Line Of Sight ◽

Interstellar Clouds ◽

Absorption Studies ◽

Low Latitudes ◽

Diffuse Clouds

AbstractThe abundance of 21-cm absorption lines seen in surveys at high latitudes can be translated into a line of sight abundance of clouds vs. column density using an empirical relationship between temperature and optical depth. As VLA surveys of 21-cm absorption at low latitudes are now becoming available, it is possible to study the variation of this function with galactic radius. It is interesting to compare the abundance of these diffuse atomic clouds (with temperatures of 50 to 100 K and masses of 1 to 10 M⊙) to the abundance of molecular clouds. To do the latter we must make assumptions about cloud cross-sections in order to convert the line of sight abundance of diffuse clouds into a number per unit volume, and to convert from cloud column density to mass. The spectrum of diffuse clouds matches fairly well the spectrum of molecular clouds, although observationally there is a gap of several orders of magnitude in cloud mass. Optical absorption studies also agree well with the 21-cm results for clouds of column density a few times 1020 M⊙.

Download Full-text

Molecular Formation in Hot Diffuse Clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900072685 ◽

1980 ◽

Vol 87 ◽

pp. 273-280

Author(s):

A. Dalgarno

Keyword(s):

Gas Phase ◽

Interstellar Clouds ◽

Gas Phase Chemistry ◽

Molecular Formation ◽

Phase Chemistry ◽

Diffuse Clouds

A description is given of the processes of molecular formation and destruction in diffuse interstellar clouds and detailed models of the clouds lying towards ζ Ophiuchi, ζ Persei and o Persei are used to assess the validity of gas phase chemistry. Modifications that may arise from shock-heated regions are discussed.

Download Full-text

Formation of Molecular Hydrogen in Interstellar Space

International Astronomical Union Colloquium ◽

10.1017/s0252921100118512 ◽

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.

Download Full-text