Interstellar clouds, carbon chemistry

H2C0, HCN, HC0+, HCC, C3N, C4H. The high density and temperature of black interstellar clouds facilitates a richer chemistry in which molecules such as dimethyl ether and ethyl alcohol are formed [1]. Figure 1 summarizes the carbon compounds which have been found in interstellar space and their abundances relative to hydrogen. Note that the carbon compounds decrease in abundance with increasing complexity. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

After these more general comments, we would like to discuss within the context of recent laboratory data some of the progress which has l n made specifically in the area of complex molecules such as cyanopolyynes. The interstellar carbon chemistry in dense molecular clouds (n 10 cm ) is used as an example. 

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-... 

The interstellar medium constitutes 10 % of the mass of the galaxy. It can be subdivided into environments with very low-density hot gas, environments with warm intercloud gas, and regions with denser and colder material (23). H and He gas are the major components of interstellar clouds molecules and submicron dust particles are only present in small concentration (22). Through gas phase reactions and solid-state chemistry, gas-grain interactions can build up complex organic molecules. Silicate and carbon-based micron-sized dust particles provide a catalytic surface for a variety of reactions when they are dispersed in dense molecular clouds 24). In cold clouds such dust particles... 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

Another class of carbon allotropes was discovered in 1985 by Harold W. Kroto, James R. Heath, Sean O Brien, Robert Curl, and Richard Smalley. Soccer-ball-shaped spheres of 60 carbon atoms with formulas like Cgg and C were found in carbon soot and later recognized to be ubiquitous in interstellar clouds. Cg is recognized as the most perfectly spherical known molecule. Because the arrangements of the carbon atoms resemble the architecture of geodesic domes, which were invented by Richard Buckminster Fuller, this class of carbon allotropes came to be cdXXed fullerenes. Kroto, Curl, and Smalley shared the 1996 Nobel Prize in chemistry for this discovery. 

ABSTRACT. Recent work on radiative processes and collisional excitation in molecular Hydrogen and its deuterated isotopic substitute and in molecular Carbon is reviewed. Particular attention is drawn to non-adiabatic coupling effects on the intensities of Lyman and Werner band systems of the vacuum ultraviolet spectrum of Hj and to the role of nuclear spin on ortho-para transitions in Hj due to collisions. The inter-relation between those processes and state to state chemistry is stressed out. We discuss the implications of these new data in a recent comprehensive model of diffuse interstellar clouds (Viala et al., 1987). 

The carbon chennistry differs from that of oxygen and nitrogen in that the ionization potential of carbon is less than 13.6 eV, so that carbon can be ionized by the interstellar radiation field. Thus carbon exists mostly as in diffuse clouds. However, the reaction between C and H2 to form CH is endothermic by about 0.4 eV, and does not proceed at low temperatures. The carbon chemistry is therefore thought to be initiated by the relatively slow radiative association reaction (Black and Dalgarno 1973)... 

In this review we have attempted to show that the circumstellar envelopes of cool, late-type stars possess a rich chemistry which is similar in many respects to that occurring in interstellar clouds. In carbon-rich envelopes, cosmic-rays and ultraviolet photons drive a chemistry dominated by ion-molecule reactions and photo-reactions. Such a chemistry has been applied to the envelope of IRC-l-10216 and has been shown to reproduce the observations extremely well. In oxygen-rich envelopes these processes also occur but the presence of large amounts of OH make neutral chemistry more important. In both cases the effects of ion-dipolar collisions has little effect on abundances, with the exception of HC3N and some protonated species (Glassgold et al. 1987, Millar 1987, unpublished). 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

Before 1985, six crystalline forms of carbon were known two forms of graphite, two forms of diamond, and chaoit and carbon (VI) discovered in 1968 and 1972, respectively. In addition a number of almost pure amorphous forms exist, such as polyacetylene (7.60) and cumulene (7.61) and recently a number of interesting nanostructured forms of carbon have been produced (Section 15.8). The year 1985 marked the discovery of the fullerenes, which represent the only truly pure molecular form of carbon, are produced under very extreme conditions as carbon vapour condenses in an atmosphere of an inert gas such as helium. Harold Kroto s interest in this chemistry originated with microwave spectroscopic studies of the atmosphere of stars and interstellar dust clouds. Kroto wanted to try to reproduce in the laboratory spectra of carbon... 

What effect do shocks have on the gas phase synthesis of complex interstellar molecules This question has been investigated at least for hydrocarbons through six carbon atoms in complexity by Mitchell (1983, 1984). He has found that if a shock passes through a dense cloud where much of the carbon is already in the form of carbon monoxide, complex hydrocarbons are not formed in high abundance. However, if a shock passes through a diffuse cloud, of density approximately 103 cm-3, where much of the cosmic abundance of carbon is in the form of C+ and to a lesser extent C, a different scenario is present. As the shock cools, the C+ and C, which remain in appreciable abundance for up to 10s yrs after the shock passage, react via many of the reactions discussed above as well as others to produce a rich hydrocarbon chemistry. The net effect is that large abundances of hydrocarbons build up as the cloud cools and eventually reaches a gas density of 3 x 104 cm-3. Do these results bear any relation to the results obtained from ambient gas phase models In both types of calculations, hydrocarbon chemistry appears to require the presence of C+ and/or C both to synthesize one-carbon hydrocarbons such as methane and then, via insertion reactions, to produce more complex hydrocarbon species. Condensation reactions do not appear to be sufficient. 

We will not mention effects on molecular formation due to shocks and shock fronts in dense molecular clouds, nor will we discuss the chemistry of the cir-cumstellar environment, where an abundance of molecular species has been detected during the past several years. In the warm, dense envelopes of stars the abundances can be matched by chemical-equilibrium calculations, in contrast to the chemical reactions which can take place in the cold interstellar molecular clouds. For example theoretical calculations based on chemical equilibrium have been performed for the expanding molecular envelope of the cool carbon star IRC H-10216 by McCabe et al. (1980), in agreement with the observed molecular column-densities. 

Before discussing the possibility of gas-phase cyanopolyyne chemistry, it seems necessary to summarize the present status of carbon chain molamle detections in interstellar space. Table 7 presents an overview of where these molecules are found and their respective abundances. These tables are an abbreviated update from Table 1 taken from Winnewisser and Walmsley (1979). It is seen that these molecules are found essentially in every type of molecular cloud from the cold dark cloud to the warm circumstellar environment, underlining the trend which has been observed over the past few years namely that complex organic molecules are not limited to a few sources only (in particular to the galactic center sources) but that they are spread over sources with rather different physical conditions. A qualifying statement may be in order here. 

However, the present discussion pertains to dark cloud chemistry. The experimental interstellar observations clearly indicate that the distribution of carbon chain molecules is correlated, and that the column densities of the longer chain members decreases about linearly with increasing chain length. Several mechanisms have been proposed for the chain building. For cool dark clouds Churchwell et al. (1978) and in further detail Walmsley et al. (1979) have proposed a formation scheme by which the longer chain molecules are formed via the acetylene backbone reaction ... 

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