Dense interstellar clouds

There are, however, still some unsolved, important questions103 about this fundamental reaction, of great relevance in dense interstellar clouds, where it could well be the source of the C3H and C3 species detected there. These unsolved questions are (a) what is the branching ratio for formation of the two isomeric C3H species, Z-C3H and C-C3H, and how does it vary with Ec (b) How do the detailed reaction dynamics and the branching ratio for C3H and C3 formation vary with Ec ... 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

The reactants in 20 can also produce the higher energy isomer HOC+, which is detected in dense interstellar clouds,37 albeit with a lower abundance than HCO+ due mainly to the catalytic reaction,... 

It is possible that electrons are not the major carrier of negative charge in dense interstellar clouds. It has been suggested that if a large fractional abundance of polycyclic aromatic hydrocarbons (PAHs) exists throughout dense interstellar clouds (see Section I), then electron sticking reactions of the type,... 

Since the extent of neutral-neutral chemistry in dense interstellar clouds is currently unclear, we have constructed three different interstellar models according to the extent of neutral-neutral reactions incorporated in them.62 Our normal model, referred to as the new standard model, does not have a significant number of atom/radical-stable neutral reactions. Ironically, this model still shows the best... 

The formation of stars in the interiors of dense interstellar clouds affects the chemistry of the immediate environment in a variety of ways depending on many factors such as the stage in the evolution of star formation, the mass of the star or protostar, and the density and temperature of the surrounding material. In general, the dynamics of the material in the vicinity of a newly forming star are complex and show many manifestations. Table 3 contains a list of some of the better studied such manifestations, which tend to have distinctive chemistries. These are discussed individually below. 

Star formation and the formation of star systems with planets around them, constantly takes place in dense interstellar clouds. The material present in these clouds is incorporated into the objects that are formed during this process. Pristine or slightly altered organic matter from the cloud from which our solar-system was formed is therefore present in the most primitive objects in the solar system comets, asteroids, and outer solar-system satellites. Pieces of asteroids (and perhaps comets) can be investigated with regards to these components through the analyses of meteorites (and eventually in samples returned from these bodies by spacecraft) in laboratories on Earth. The infall of asteroid and comet material from space may have contributed to the inventory of organic compounds on primordial Earth. 

Blake GA, Keene J, Phillips TG (1985) Chlorine in Dense Interstellar Clouds The Abundance of HC1 in OMC-1. Astrophys J 295 501... 

Morris, M., Turner, B. E., Palmer, P. Zuckerman, B. 1976 Cyano acetylene in dense interstellar clouds. Astrophys. J. 205, 82-93. 

The sun, the planets and satellites like the moon were formed 4.5 Gyr ago as a consequence of gravitational instability in a part of a dense interstellar cloud. This particular dense insterstellar cloud no longer exists for obvious reasons, but other dense interstellar clouds can still be observed in our galaxy. Some of these dense... 

As we said above, grains and dust particles constitute a major component of dense interstellar clouds. Their role in the synthesis of complex molecules is accepted by many authors, but the relative proportion of gas-phase to on grain reactions is at the centre of an interesting debate. All authors agree on the necessary role of on grain reactions in the synthesis of the dihydrogen molecule, the most abundant molecule in the Universe. To return to the role of dust particles in the synthesis... 

Both of these approaches involve collisions of two large species. According to model calculations by one of us (Herbst 1983), neither of these syntheses can reproduce the observed high abundance of HC3N in sources such as the nearby dense interstellar Cloud TMC-1. However, this negative assessment for the first mechanism relies on a laboratory rate coefficient for reaction (3.39) at room temperature that shows the reaction to be slow slow ion-molecule reactions at room temperature often become more rapid at lower temperature (Rowe et al. 1984). Likewise, the negative assessment for the second mechanism is based on a theoretical calculation for the rate coefficient of the radiative association reaction. 

The problem of the existence of a gas phase in dense interstellar clouds arises because the time scale for condensation onto grains appears to be shorter than the life time of these regions and, until recently, it has not been felt by many investigators that there was a viable mechanism for returning molecules heavier than H2 back into the gas. 

Within a distance of l kpc from the sun, dense interstellar clouds can be observed optically because they attenuate the light of stars located behind them. Fig. 10 shows as an example the dust clouds Barnard 1968 and 1972 which are seen as dark areas in an otherwise rather uniform field of stars. Fig. 11 shows another dust cloud, the horsehead nebula", silhouetted against the bright background of an emission nebula (HII region). Most of the extended dust clouds... 

Typical kinetic gas temperatures in dense interstellar clouds are 7k < 30 °K but may in some cases reach values of v 80 °K (e. g. the Orion molecular cloud). The relation between excitation temperature Tex and kinetic gas temperature 7k is further discussed in Sections III. E and III. F. 

The aim of this article is to give a short outline of current theories of molecule formation and destruction in interstellar clouds, together with a short summary of the observational material which has been accumulated up to early 1981. Although this article will address itself predominantly to simple molecules a section on complex molecules has been added. We will, therefore, discuss some general aspects of cosmochemistry and then turn to molecule formation in diffuse clouds followed by a discussion of the chemistry of dense interstellar clouds. A section has been added to summarize recent observational results and theoretical proposals in understanding the formation of intermediate and complex molecules, an area of considerable current activity. Finally the article closes with a short summary of the molecular species found in planetary atmospheres and a short discussion of what the relation might be to the interstellar molecules. 

Reactions of C with H, H2 are of fundamental importance to the carbon chemistry in interstellar clouds, and some of the reaction paths prevalent in dense interstellar clouds may also be of significance in the reducing atmospheres of the outer planets. These reactions initiate a complex sequence which produce CH, CH and lead eventually to molecules such as CH, These molecules are important pre-... 

Leung, C.M. Herbst, E. Huebner, W.F. Synthesis of complex molecules in dense interstellar clouds via gas-phase chemistry — A pseudo time-dependent calculation, Astrophys. J. Suppl. 1984, 56, 231-256. 

Herbst, E. and Klemperer, W. (1973). The formation and depletion of molecules in dense interstellar clouds. Astrophysical Journal, 185, 505. 

In the cold and dense interstellar clouds, cooling is dominated by the rotational transitions of the abundant CO molecule and its isotopomers. CO possesses a spectrum of lines due to the rotation of the molecule. The permitted energy levels are approximately those allowed by quantum mechanics for a rigid rotator, i.e. a linear molecule that does not change shape as it rotates... 

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