Interstellar Dust Clouds Research Articles

Gravitational collapse in dusty plasmas with mass distributions and neutral gas

Jeans and two-stream instabilities are revisited for mixtures of charged and neutral dust, and also for different species of charged dust. From the general dispersion law for different dust species, limiting cases of monodisperse or polydisperse but cold dust components are discussed, putting results from the literature in perspective. Dispersion laws and critical Jeans lengths are then derived for a mixture of two dust species, or of charged and neutral dust, the latter being of interest in interstellar dust clouds where not enough electrons are available to charge all the grains. Instabilities are considered for two counterstreaming dust beams of equal strength, showing how the Jeans and streaming instabilities are coupled in specific wave number domains.

A new scheme of radiation transfer in HII regions including transient heating of grains

A new scheme of radiation transfer for understanding the infrared spectra of HII regions, has been developed. This scheme considers non-equilibrium processes (e.g. transient heating of the very small grains, VSG; and the polycyclic aromatic hydrocarbon, PAH) also, in addition to the equilibrium thermal emission from normal dust grains (BG). The spherically symmetric interstellar dust cloud is segmented into a large number of “onion skin” shells in order to implement the non-equilibrium processes. The scheme attempts to fit the observed SED originating from the dust component, by exploring the following parameters: (i) geometrical details of the dust cloud, (ii) PAH size and abundance, (iii) composition of normal grains (BG), (iv) radial distribution of all dust (BG, VSG & PAH).

Complex chemical reactions — A review

Complex responses have been observed in a wide range of chemical and engineering systems: In well-stirred and unstirred solution-phase, in gels, in heterogeneous catalysis and dissolution reactions, in gas- and solid-phase combustion, widely in biological systems, accompanying phase transitions, in atmospheric kinetics and even in interstellar dust clouds. Complex behaviour does not, however, imply a necessarily complex underlying chemical mechanism. The appropriate feedback mechanisms, built on chain-branching, autocatalysis or self-heating, arise quite commonly in Nature. In many instances, it is appropriate first to attempt to find reduced mechanisms giving a semi-quantitative fit to observed responses. An example of such an approach applied to a model for complex oscillations of species concentrations in the mesosphere is presented, and the reduced model used to investigate the effects of longitudinal mixing in this system. Once a general understanding of a particular reaction system has been obtained, more detailed questions can be addressed. This is exemplified by a study of the development of three-dimensional scroll waves in the Belousov–Zhabotinsky reaction. Such detailed information for a specific reaction also informs the general understanding of the class of `excitable media'.

Infrared emission from interstellar dust cloud with two embedded sources: IRAS 19181 + 1349

Mid-and far-infrared maps of many Galactic star forming regions show multiple peaks in close proximity, implying more than one embedded energy source. With the aim of understanding such interstellar clouds better, the present study models the case of two embedded sources. A radiative transfer scheme has been developed to deal with a uniform density dust cloud in a cylindrical geometry, which includes isotropic scattering in addition to the emission and absorption processes. This scheme has been applied to the Galactic star forming region associated with IRAS 19181 + 1349, which shows observational evidence for two embedded energy sources. Two independent modelling approaches have been adopted, viz., to fit the observed spectral energy distribution (SED) best; or to fit the various radial profiles best, as a function of wavelength. Both the models imply remarkably similar physical parameters.

Extinction Curves, Distances, and Clumpiness of Diffuse Interstellar Dust Clouds

We present CCD photometry in UBVRI of several thousand Galactic field stars in four large (>1 deg2) regions centered on diffuse interstellar dust clouds, commonly referred to as clouds (with optical depth AV less than unity). Our goal in studying these stars is to investigate the properties of the cirrus clouds. A comparison of the observed stellar surface density between on-cloud and off-cloud regions as a function of apparent magnitude in each of the five bands effectively yields a measure of the extinction through each cloud. For two of the cirrus clouds, this method is used to derive UBVRI star-count–based extinction curves, and U-band counts are used to place constraints on the cloud distance. The color distribution of stars and their location in (U-B, B-V) and (B-V, V-I) color-color space are analyzed in order to determine the amount of selective extinction (reddening) caused by the cirrus. The color excesses, Aλ-AV, derived from stellar color histogram offsets for the four clouds, are better fitted by a reddening law that rises steeply toward short wavelengths [RV ≡ AV/E(B - V) 2] than by the standard law (RV = 3.1). This may be indicative of a higher than average abundance of small dust grains relative to larger grains in diffuse cirrus clouds. The shape of the count-based effective extinction curve and a comparison of different estimates of the dust optical depth (extinction optical depth derived from background star counts/colors; emission optical depth derived from far infrared measurements) are used to measure the degree of clumpiness in clouds. The set of techniques explored in this paper can be readily adapted to the Sloan Digital Sky Survey data set in order to carry out a systematic, large-scale study of cirrus clouds.

Modeling Extinction and Infrared Emission from Fractal Dust Grains: Fractal Dimension as a Shape Parameter

In astrophysical environments, dust grains are most likely formed by stochastic growth processes, e.g., particle aggregation or cluster aggregation, and should have irregular shapes. Yet spherical grains are often assumed in astrophysical models. We have constructed detailed radiation transport models to study the effects of nonspherical dust grains on the spectra of circumstellar envelopes and interstellar dust clouds. In general, nonspherical grains have larger absorption cross sections at long wavelengths. Being less compact and having lower ratios of volume to surface area, they are generally cooler than their spherical counterparts, typically by 10%-20%. Hence radiation transport models of infrared sources with nonspherical grains would show a shift in the peak flux toward longer wavelengths. Furthermore, the flux ratio of 10 μm to 20 μm silicate emission features decreases for the less compact nonspherical grains. We have also examined the effects of grain shape on a standard model of the interstellar extinction curve. In addition, we find that the error introduced by using bulk optical constants for submicron particles is significantly smaller than that introduced by assuming spherical grains. We have identified, for the first time, fractal dimension D as a shape parameter characterizing the optical, thermal, and radiative properties of dust grains. The fractal dimension D depends on the ratio of volume to surface area and is defined by the relation N(r) ~ rD, where N(r) is the number of monomers within a sphere of radius r. In general 1 < D < 3, and a smaller D implies a more filamentary and less compact grain morphology. For a given mass, grains with the same D, independent of detailed shape, show no significant difference in their absorption cross sections, temperatures, and energy spectra. Hence in modeling phenomena involving irregularly shaped grains, we need to introduce just one parameter, the fractal dimension D, to characterize the shape. There are several important astrophysical implications of removing the unrealistic assumption of spherical grains: (1) determination of dust column density based on models assuming spherical grains would lead to an overestimate; (2) models of the interstellar extinction curve using fractal grains instead of spherical grains would require less elemental depletion, typically by one-third; and (3) in the study of radiation-driven mass loss in evolved stars, both the mass-loss rates and details of outflow dynamics may need to be revised since radiation pressure on dust depends sensitively on the extinction cross sections of newly formed grains.

Far infrared multi-frequency observation of interstellar dust clouds

Diffuse interstellar dust emission at wavelengths between 0.5 and 2 mm has been detected by the ARGO 1993 balloon borne telescope. The instrument scanned the Galactic plane in Aquila and several cirrus clouds in Aries, Taurus and Hercules. The data are spatially consistent with the IRAS 100 μm sky maps, and the spectral behaviour is consistent with a single temperature thennal spectrum with T d ∼ 21 K and a spectral index of emissivity α ∼ 1.5.

Infrared cirrus, what have we learned?

IRAS discovered thermal emission from infrared cirrus in 1983 and COBE has extended observations of these interstellar dust clouds to additional IR wavelengths. IR studies of individual high galactic latitude clouds are combined with sensitive optical measurements of scattered light from the same clouds. At least 4 distinctly different types of grains are required to explain the complex relationships revealed by combining observations of various samples of clouds made on different spatial scales and in a number of spectral bands. In addition to the dominant 22 K component seen most prominently at 100 μm, many clouds are bright at 12 and 25 μm, indicating dust at temperatures well above 100 K. Optical studies suggest at least two more types of grains are needed to explain the measured blue and red colors and the highly variable ratio of blue light to l00 μm thermal emission that is observed. We show a new display of 100 μm cirrus on a global scale and we determine the power density in a typical cloud to spatial scales of a few arcsec.

Further measurements on isotopic anomalies of 196Hg/202Hg ratio in some of acid insoluble residues of Sikhote Alin and other iron meteorites.

In residues of several iron meteorites, obtained by dissolution of a metal piece in 2M H2 SO4, isotopic ratio of 196Hg/202Hg has been measured by radiochemical neutron activation analysis in Hg fractions, released by stepwise heating. Both positive and negative deviations relative to terrestrial composition, have been found. Anomalies are more frequently observed in the high temperature fractions (200 < T < 600°C) as compared to the low temperature distillates (T ≤ 200°C). The largest deviations so far seen are, respectively, enrichment of 196Hg by 67% in a 400°C fraction of Carlton and depletion of the same isotope by 86% in a 300°C fraction of Sikhote Alin residue (magnetic) of <38 μm grain size. The anomalous components appear to be sporadic and heterogeneously distributed. A lack of homogenisation of the two isotopes that are produced in different nucleosynthetic processes suggests that iron meteorites have been formed in a non-magmatic process, entrapping presolar grains. Alternatively, some foreign materials presumably of different isotopic compositions might have been introduced in meteorite parent bodies during their collisions with other cosmic objects and or dense cloud of interstellar dust.

Radiation transport in dust in disk geometry. I - Application to externally heated interstellar clouds

view Abstract Citations (23) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation Transport in Dust in Disk Geometry. I. Application to Externally Heated Interstellar Clouds Spagna, George F., Jr. ; Leung, Chun M. ; Egan, Michael P. Abstract The dust temperature distribution and the infrared energy spectrum of disk-shaped interstellar dust clouds which are heated externally by the ambient interstellar radiation field are determined by solving numerically the radiation transport problem for both 1D and 2D disk geometries. The effects of source geometry and cloud opacity on the dust temperature distribution, radiation field anisotropy, and infrared energy spectrum are investigated. It is found that equilibrium grain temperature is not extremely sensitive to the cloud geometry or degree of disk flattening. Compared to a spherical model of the same opacity, a similar disk model shows a nearly identical temperature distribution and emergent flux spectrum. The internal radiation in the far-infrared is found to be more anisotropic in disk models and to be predominantly radially streaming. Publication: The Astrophysical Journal Pub Date: September 1991 DOI: 10.1086/170497 Bibcode: 1991ApJ...379..232S Keywords: Cosmic Dust; Infrared Spectra; Interstellar Matter; Interstellar Radiation; Nebulae; Radiative Transfer; Anisotropy; Energy Spectra; Mathematical Models; Numerical Analysis; Opacity; Radiant Flux Density; Temperature Distribution; Astrophysics; INFRARED: SPECTRA; INTERSTELLAR: GRAINS; NEBULAE: GENERAL; RADIATIVE TRANSFER full text sources ADS |

Infrared emission from isolated dust clouds in the presence of very small dust grains

We have developed a continuum radiation transport code which incorporates the transient heating of small dust grains. Using this computer code, we have constructed models to study the effects of the temperature fluctuations due to small grains on the infrared spectrum and surface brightness of externally heated interstellar dust clouds. From comparison of the model results with IRAS observations, we conclude that the observed emission in the 12- and 25-μm bands is most likely produced by small grains with radius of ∼10 Å, while the 60- and 100-μm emission comes primarily from large grains heated under the equilibrium conditions. Our models, based on the assumption of a constant fractional abundance of large and small grains throughout a cloud, predict infrared limb brightning only for models with a visual optical depth ⪆ 20. The fact that relatively strong limb brightening has been observed for more diffused globules indicates that the small grain abundance is likely to be increased in outer parts of these objects.

Hg isotopes on the moon and in achondrites

196Hg and 202Hg isotopes measured by neutron activation analysis have been found to have ratios differing from normal in some lunar and meteoritic matter. Some Hg fractions extracted by stepwise heating from several samples of Apollo- 14 breccia 14305 and one sample of 14321 exhibit deviations from the normal isotopic ratio by ∼ 20–30%. Samples from three Antarctic basaltic achondrites contain 196 Hg 202 Hg ratios that are lower than normal by factors of 2–4. The anomalies in these meteorites and the temperature fractions in which they occur are reproducible. The anomalous Hg occurs in a single but variable temperature fraction from 14305 samples but only in the 130°C fraction from the three achondrites; data on two of the Antarctic achondrites have been published. The amount of Hg in the fractions with anomalous ratios varied from 10% to ∼ 0.1 % in samples with total Hg of 2–200 ppb. The other 5 to 6 thermally extracted Hg fractions per sample have normal ratios. These fractions act as internal standards and thus serve to reduce uncertainties related to neutron flux variables and sample matrix effects. The anomalous Hg is found in samples from three possible parent bodies: the Moon, that of the shergottites and that of the eucrites. The final thermal evolution of the samples occurred over a time-span ranging from < l.3 to ⩾ 3.9 Ga ago. These facts present obstacles to understanding how this anomalous Hg was acquired; a possible explanation is periodic encounters of the solar system with a dense cloud of interstellar dust containing such Hg.

What is Happening at the Center of Our Galaxy?

Recent discoveries and new technologies have made it possible at last to study the center of our own galaxy in some detail. These developments include improved techniques for collecting analyzing astronomical radio waves and infrared radiation as well as space flights above the earth's atmosphere, which have made possible the detection of energetic X-ray and gamma-ray radiation emanating from the center of the galaxy. All these types of waves--radio, infrared, X-ray and gamma-ray--are similar to visible light in that they are all forms of electromagnetic radiation and differ only in their wavelengths and energy levels. Unlike light, however, they can penetrate interstellar clouds of dust with some ease and hence provide a window into the structure and dynamics of the galactic center. Each wavelength region probes different aspects of the physical properties there; for example, X rays are emitted by very hot gas, whereas much infrared radiation is emitted by interstellar dust grains and cooler gases. Studies at these various wavelengths have produced a wealth of information about and new understanding of the central part of our galaxy. They have also raised a host of new puzzles. This mysterious region contains antimatter, radioactive clouds, veiled sources of intense radiation, swirlingmore » gas and dust and an unseen object with a tremendous gravitational pull--perhaps a massive black hole.« less

Concluding remarks

This discussion on the origin of the Solar System has quite rightly focused on the chemical evidence. Harold C. Urey was fond of quoting Edward Teller’s remark about the atomic bomb: ‘ Its physical effects disappear quickly but its chemical effects are lasting’. The chemical evidence, particularly the abundance of elements and isotopes, obtained from meteorites, by ground-based astronomical methods and from space missions continues to be the starting point for inferring the processes by which the Solar System came into being and reached its present configuration. When the origin of the Solar System was first discussed it appeared a natural assumption that the material from which the planets were made came from the Sun. Similarly before the importance of radioactivity in the Earth was recognized it seemed obvious that the planets originated as liquid drops by condensation from gases drawn from the Sun. The fact that the Earth’s gravity field, for example, was almost exactly that of a hydrostatic body in equilibrium, under its self-gravitation and the centrifugal forces due to rotation, made this assumption a natural starting point. It was the discovery of the interstellar dust clouds by astronomers that first provided evidence for a nebula theory. We have been correctly reminded in this discussion that the idea of a close approach of another star from which material was drawn out by the Sun is still a possible theory, and it is entirely in the interest of a scientific approach that it should be further investigated so that its predictions can be more thoroughly tested.

Physicochemical Isotope anomalies

Isotopic composition of refractory elements can be modified, by physical processes such as distillation and sputtering, in unexpected patterns. Distillation enriches the heavy isotopes in the residue and the light isotopes in the vapour. However, current models appear to be inadequate to describe the detailed mass dependence, in particular for large fractionations. Coarse- and fine-grained inclusions from the Allende meteorite exhibit correlated isotope effects in Mg both as mass-dependent fractionation and residual anomalies. This isotope pattern can be duplicated by high temperature distillation in the laboratory. A ubiquitous property of meteoritic inclusions for Mg as well as for most of the other elements, where measurements exist, is mass-dependent fractionation. In contrast, terrestrial materials such as microtektites, tektite buttons as well as lunar orange and green glass spheres have normal Mg isotopic composition. A subset of interplanetary dust particles labelled as chondritic aggregates exhibit excesses in 26Mg and deuterium anomalies. Sputtering is expected to be a dominant mechanism in the destruction of grains within interstellar dust clouds. An active proto-sun as well as the present solar-wind and solar-flare flux are of sufficient intensity to sputter significant amounts of material. Laboratory experiments in Mg show widespread isotope effects including residual 26Mg excesses and mass dependent fractionation. It is possible that the 26Mg excesses in interplanetary dust is related to sputtering by energetic solar-wind particles. The implication of the laboratory distillation and sputtering effects are discussed and contrasted with the anomalies in meteoritic inclusions and other extraterrestrial materials we have access to.

Numerical solution of the radiation transport equation in disk geometry

We describe an efficient numerical method for solving the problem of radiation transport in a dusty medium with two dimensional (2-D) disk geometry. It is a generalization of the one-dimensional quasi-diffusion method in which the transport equation is cast in diffusion form and then solved as a boundary value problem. The method should be applicable to a variety of astronomical sources, the dynamics of which are angular-momentum dominated and hence not accurately treated by spherical geometry, e.g. protoplanetary nebulae, circumstellar disks, interstellar molecular clouds, accretion disks, and disk galaxies. The computational procedure and practical considerations for implementing the method are described in detail. To illustrate the effects of 2-D radiation transport, we compare some model results (dust temperature distributions and i.r. flux spectra) for externally heated, interstellar dust clouds with spherically symmetric and disk geometry.

A Historical Review of Star Formation: Observation and Theory

We present a historical review of evidence for ongoing star formation in our Galaxy beginning with the discovery that interstellar space is not empty. The discoveries of interstellar dust, interstellar hydrogen and molecular clouds are reviewed. Observational investigations of dark clouds are then traced from the photographs of Edward Emerson Barnard to contemporary studies of their molecular constituents. A historical overview of observational evidence for new-born stars includes T-Tauri stars, young stellar clusters, sequential star birth and infrared stars beginning with Alfred Joy, Merle Walker, Becklin, and Neugebauer, and Adrian Blaauw and continuing to giant molecular clouds and IRAS. Theoretical studies of gravitational collapse and the early stages of stellar evolution are also placed within a historical context.

Comets and life

Some of the chemical species which have been detected in comets include H 2O, HCN, CH 3CN, CO, CC 2, NH 3, CS, C 2 and C 3. All of these have also been detected in the interstellar medium, indicating a probable relationship between interstellar dust and gas clouds and comets. Laboratory experiments carried out with different mixtures of these molecules give rise to the formation of the biochemical compounds which are necessary for life, such as amino acids, purines, pyrimidines, monosaccharides, etc. However, in spite of suggestions to the contrary, the presence of life in comets in unlikely. On the other hand, the capture of cometary matter by the primitive Earth is considered essential for the development of life on this planet /1/. The amount of cometary carbon-containing matter captured by the Earth, as calculated by different authors, is several times larger than the total amount of organic matter present in the biosphere (10 18g). The major classes of reactions which were probably involved in the formation of key biochemical compounds are discussed. Our tentative conclusions are that: 1) comets played a predominant role in the emergence of life on our planet, and 2) they are the cosmic connection with extraterrestrial life.

Does multiclustering of galaxy aggregates appear as a specific coil of extragalactic data unseparated from interpretations of the role of interstellar obscuration?

In the Jagiellonian Field (JF) the galaxies form themselves into a type of Pallium embedding the so-called Central Void. The popular suggestion was that the Jagiellonian Pallium/Patchy Structure (JPS) is caused by real existence of genuinely empty volume of intercluster space or — in terms of the Estonian School hypothesis — of intracellular space. Interstellar obscuration was never considered as an explanation of JPS, probably because of cosecant-type law which makes galactic extinction improbable; secondly, because of Zwicky and Rudnicki suggestions that the region of sky is free of selective extinction, and thirdly because of tradition (specific ‘problem-situation’) in extragalactic astronomy that clusters of galaxies form real — not false — extended galaxy structures and multiclustering is not produced by dust obscuration. It is shown that a small and relatively dense cloud, at galactic latitudeb=72°, of galactic dust contributing to the average extinction about 0 . 01 is projected just into the region of the Central Void. Interstellar obscuration affects multiclustering phenomenon in more serious degree than it can be theoretically predicted and/or reconstructed from studied distribution of stars. We confirm the view of Zieba that hierarchy of interstellar clouds of dust must lead to hierarchical clustering of galaxies.

A workshop on dusty plasmas

“Dusty plasmas” is the name given to plasmas heavily laden with charged dust grains that, together with the surrounding ions and electrons, constitute a new kind of plasma regime. Such plasmas exist in the rings of planets, in the atmospheres of comets, and in interstellar dust clouds. The dust grains are strongly coupled to the fields and particles in the plasma through their electrical charges. As a result, the motions of the grains may be dominated by electric and magnetic forces, and collective effects can play an important role. This field of study is receiving increased attention because of the observations of dust during recent spacecraft missions to the planets and comets, together with the dawning recognition that the evolution of dusty plasma clouds in space may be quite different from that of nondusty clouds.