Dark cloud

On edge along 153, falls in infrared minimum at edge of Bar. 7 S of N76 in extended emission in NH Bar. 

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The regions of space where molecules have been detected are the nebulae which are found not only in our own galaxy but also in other galaxies. In our galaxy the nebulae are found in the Milky Way, which appears as a hazy band of light as a result of its containing millions of stars. Associated with the luminous clouds composing the nebulae are dark clouds of... 

The rotational spectrum has been calculated accuratly by ab-initio methods [2], and has been measured in the laboratory with high precision [3,4], so that the radio detection of C3H2can be done without ambiguity, encouraging its search in different environments as dense dark clouds [5], diffuse interstellar medium [6] or Hll regions [7]. 

Aye, my lord was all they vouchsafed, but it was as well they were taciturn, for a qualm of hope had seized me. To the west, dark clouds were massing, but also, down Alley Yard, I had seen Louis. He made no sign that might have betrayed him or me, and even as I turned my head away that my gaze might not draw others to him, he vanished. 

The mean particle density of ISM is 106 particles per cubic meter there are, however, great variations from this mean value. Between the spiral arms of the Milky Way, there are between 104 and 105 hydrogen atoms per cubic metre in the dark clouds and the HII regions, there are 10s—1010. Up to 1012—1014 hydrogen atoms per cubic metre are present in regions with OH sources and in certain infrared objects. 

The dark clouds were responsible for the discovery of ISM, as they absorb the light from stars which lies behind these clouds of interstellar matter. It is difficult to obtain reliable information on the dust particles. They are probably about 0.1 pm in diameter, consisting of a silicate nucleus and an envelope of compounds containing the elements C, O and N, which, with H and He, are the main elements present in interstellar space. There are only two sources of information for more exact characterisation of the dust particles ... 

These different classes of molecules reflect the variety of reaction conditions which are present in ISM. Thus ion-molecule reactions dominate in the dark clouds these reactions are driven by ionisation processes and lead to unsaturated molecules, ions and radicals. 

However, one of the electrical engineers at the University of California, Jack Welch, was willing to work with me, and I could use his radio antenna. So I had a student, Albert Cheung, take a look, and he looked at these dark clouds in space, and sure enough, there was ammonia. Well, since we found ammonia, we thought, we ought to look for water too, just to try this out. So the student looked, and there was water radiation. In fact it was very intense - hey, it had to be a maser, maser amplification in interstellar space And OH had already seemed to indicate something similar. 

The TMC-1 is a cold quiescent dark cloud some 45 ly away with a temperature of around 10 K. It is predicted to be only 100000 years old and, up to 1998, 49 molecules have been detected within it. 

There are some variations in the composition of chemical clouds as they evolve in time from dark clouds such as TMC-1 to giant molecular clouds such as Orion where the presence of light from young stars initiates photochemistry. The Orion molecular cloud chemical inventory contains several saturated species such as ethanol (CH3CH2OH) and its CN analogue (CH3CH2CN), the simplest carboxylic acid (CH3COOH, acetic acid) and methylamine (CH3 NH2). 

There are many possible chemical reactions that must be considered for all networks of chemical processes, whether within a cold dark cloud, a giant molecular... 

Photochemical reactions are initiated by the interaction of a molecule with the local radiation field, perhaps from an embedded star. In cold dark clouds the radiation field may be rather small but as stars begin to form and emit radiation, initially at low energy but of every-increasing energy, photochemistry becomes important in generating a wide variety of reactive species to add to the diversity of species available for the chemical networks. 

The legal committee met often. According to one of Von Knieriem s assistants, "The dark clouds of the Sudeten crisis were aheady appearing over the horizon. There was a general feeling of the darkening of the political situation."... 

As he hopped along, he thought he saw a few dark clouds in the distance. [ finish the story ]... 

Stars drive galactic evolution. At the end of their existence, they inseminate space with the products of their nuclear alchemy. Then, in dark clouds, sheltered from ravaging photons, molecules build up. Stars and planets are constantly being born in the cold of space. 

We give an estimation of the carbon fraction locked in these molecules. We discuss the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes in various phases of the interstellar medium. These molecules could be the carriers of the anomalous microwave emission detected by Watson et al. (Astrophys. J. 624 L89,2005) in the Perseus molecular complex and Cassasus et al. (2006) in the dark cloud LDN 1622. Hydrogenated forms of fullerenes may account for the dust-correlated microwave emission detected in our Galaxy by Cosmic Microwave Background experiments. 

In summary, it is possible to understand the Perseus molecular complex and the dark cloud LDN 1622 microwave anomalous emission in terms of electric dipole emission of fulleranes if these molecules follow a size distribution similar to that proposed in the study of the UV extinction bump. The dominant microwave emission would be associated in both cases to the smaller fulleranes. These molecules could also be responsible for the diffuse microwave dust correlated emission at high Galactic latitude detected by the COSMOSOMAS experiment and WMAP. 

The room settled. OnlyMissier Grande remained standing. The mood had changed, the dark clouds of worry rolled back to reveal the pearly sunlight of the Adriatic. It had been the foreigners all along. 

The pseudodipeptide amides represented a major advance in the substrate-based design of FTIs. They removed the metabolic liability of the prodrug, were cell active, and were considerably simpler chemically. However, the majority of pseudodipeptide amides were not potent enough and too cytotoxic to demonstrate activity in cell culture. Although the potency issue was perceived as solvable, the dark cloud of nonspecific cytotoxicity hovered over this particular class of compounds like an unwelcome visitor. Changes to the molecule which virtually destroyed FTase activity had no effect on the level of cytotoxicity, indicating that the observed toxicity was unlikely to be mechanism related. 

Dark clouds clouds with assoc. H 4 region Circumstellar ... 

Figure 1.4 The azimuthally integrated density profile of the Barnard 68 dark cloud revealed that its density structure is consistent with being in hydrostatic equilibrium. The cloud may, however, be only marginally stable (Alves et al. 2001). The radius of the cloud is 12500AU.

As long as so little is known about the interstellar particle and photon radiation fields which provide the heating of the interstellar gas, one should not put too much faith in estimated equilibrium temperatures of dense gas clouds such as shown in Fig. 9. However, at least qualitatively it appears to be correct that dense condensations in the interstellar gas have low kinetic temperatures. In fact, gas-kinetic temperatures as low as 5 °K have been observa-tionally determined in the center of dark clouds. 

Fig. 12. Contour map of the visual extinction in the Taurus cloud. The shaded areas represent dark clouds of small diameter with visual extinction ranging from 5m to 8m. Also indicated are the positions of T-Tauri stars and quasi-thermal OH emission...

Dark clouds have been catalogued most recently by Lynds (1962). Although their distances are rarely known, Hollenbach et. al. (1971), with some plausible assumptions, have estimated physical parameters for Lynds dust clouds. Their results are given in Table 4. 

Observations made with high angular resolution revealed for one of the black clouds in the nuclear disk a quasi-rigid-body rotation. Kinetic gas temperatures in black clouds appear to be higher than in dark clouds, which implies that they contain some source of heating. The average density in black clouds is of the order of 103 particles cm-3 but, in some condensations, may go as high as 10s... 

In dark clouds the dust protects the molecules from dissociation by UV radiation. This protection is presumably even better in the case of Scoville s black... 

In denser clouds, however, the abundance of H2 increases rapidly. With gas densities > 102 cm-3 and column densities corresponding to an extinction in the visible of Av > 0.5m hydrogen is expected to be mainly in the molecular form (Werner et al., 1970). Similar results are obtained by de Jong (1972) who also includes low-energy cosmic radiation in his calculations which produces electron densities in dark clouds sufficient to yield H2 formation via associative detachment H- + H -> H2 + e proceeding with a rate constant k v 1.3 x 10 9 cm3 sec-1 at 300 °K. This reaction, however, is only of importance if formation of H2 molecules on grains is for one reason or another inefficient. 

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