Circumstellar molecules

The paramount importance of carbon in the cosmos is shown by the fact that more than 75% of the approximately 120 interstellar and circumstellar molecules so far identified are carbon containing (Henning and Salama, 1998). Molecules apparently travel from the ISM via protoplanetary discs to the planetesimals and from there, via accretion, to the heavenly bodies formed. The molecules so far identified in ISM come from quite different types of compounds ... 

Table 3.3 Interstellar and circumstellar molecules (Irvine et al., 2003 Thaddeus, 2006)...

A database of interstellar and circumstellar molecules is maintained at http //www.cvrtrao.edu/ awootten/allmols.lltml by A.L. Wootton with the current total at 129 (at time of going to press). 

For carbon-rich outflows, this scheme seems to predict, reasonably well, the observed abundances and spatial distributions of some of the most abundant circumstellar molecules such as CO and HCN (Olofsson et al. 1990 Cherchneff ... 

Silicon dicarbide has been identified by Thaddeus et al. (1984) as a circumstellar molecule on the basis of 9 hitherto unassigned millimeter wave lines observed in the late type star IRC + 10216. The molecule is the first ring molecule detected in space, and its rotational spectrum is that of a near prolate asymmetric top with C2v symmetry. The molecule had been detected in the laboratory prior to the interstellar detection by optical laser spectroscopy (Michalopoulous et al. 1984). 

Table 17.1. Observed interstellar and circumstellar molecules The species listed are observed by their rotational emission spectra unless...

Many investigations of circumstellar molecules have been done for C-rich CSE. Figure 4 shows observed abundance trends of several molecules as a function of radial distance in the thick CSE of the well-studied C star IRC+10216. The top shows gases whose abundances are determined by thermochemical equilibrium and quenching near the photosphere the center panel shows molecules that seem to originate beyond 10R (within the CSE), and the bottom panel shows some of the mainly photochemically-produced gases. 

Table 2.3 List of detected interstellar and circumstellar molecules, radicals and ions, grouped by the number of atoms (N) they contain. Species detected with UV, visible or infrared spectroscopy are indicated in italics...

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. 

Carbon monoxide carbo-mers [5-7], i.e., monoxides of linear odd carbon chains longer than one C 0 n = 3, 5, 7, 9...), are highly reactive molecules suggested as potential constituents of interstellar and circumstellar gas clouds. Considerations based on MO theory and quantum chemical calculations indicate that, similar to pure odd carbon chains, all these heterocumulenes are singlet carbenes in the ground state [93]. Since its matrix isolation in 1971 [94] and its synthesis in gas phase in 1983 [95], the simplest member of this family, i.e., tricarbon monoxide C3O, has been extensively studied both experimentally and theoretically [96-102], and its interstellar presence fully confirmed [103]. In particular, on the basis of... 

A hundred or so different molecular species have been spotted in molecular or circumstellar clouds. Most interstellar molecules are organic molecules, that is, they are carbon-based. This indicates that chemical evolution does occur on a cosmic scale. What is more, many molecules on the interstellar list are fundamental building-blocks for the construction of biological structures. 

Several different types of this dust are distinguished by astronomers. On average, interstellar dust resides in widely separated diffuse clouds. But there are also dense regions of gas and dust into which little ultraviolet radiation can penetrate, thereby providing an environment for the formation of complex molecules these are referred to as molecular clouds. Clouds of particles expelled by cooler stars into the regions around them are called circumstellar... 

Water maser emission has been detected towards V778 Cyg (Fig. 1) in the present observation. It is another evidence of the oxygen-rich nature of the circumstellar envelope around V778 Cyg, because water vapor has always been found around M stars. Thermochemical calculations also support the idea that the water molecules are present in the oxygen-rich environment but not in the carbon-rich one. The positional coincidence of the water maser with the optical star was found to be less than 0.5 arcsec (Deguchi et al. 1987), (Fig. 2). Unfortunately no other emission has been found, though Benson and Little-Marenin (1987) observed a water maser in EU And. Our negative detection of maser emission in EU And was probably due to the intensity variation of the object. 

In C-rich stars almost all atoms of oxygen are blocked in CO molecules, whereas the large fraction of carbon is tied in molecules C H and CgHg. The rest of carbon exists in the form of free atoms and molecules of silicon carbide SIC. Both carbon and silicon carbide are the most abundant condensible species in C-rich stars. The presence of SiC circumstellar grains is compelling due to the ii.3 im spectral feature identified firstly by Treffers and Cohen (1974). 

The strict solution for the problem of the resistance to the motion of a small sphere moving through gas has been obtained by Baines et al. (1965). They considered both specular and diffuse reflection of the molecules at the surface of the sphere mass of which is large in comparison with the mean mass of gas molecules and the radius to be small compared with the mean free path of gas molecules. All these assumptions are applicable for circumstellar outflows. Fadeyev and Henning (1987) used these solutions for calculation of momentum transfer from silicate dust grains to gas molecules in cool 0-rich red giants... 

In 1985. similar experiments were conducted at Rice University. In a 1988 paper. Curl and Smalley (Rice University) outlined their experiments with carbon cluster beams, essentially using the clusler-generaung apparatus previously described by the Exxon researchers. Initially, this experimentation was motivated by an interest that had been shown by die astrophysicist, Krotu (University of Sussex), who had been modeling the formation of carbon molecules in circumstellar shells. As a consequence, the Rice University team concentrated its studies on the smaller (2- to 30-atom) carbon clusters. As pointed out in the Curl-Smalley paper, the objective was to determine if some or all of the species had the same form as the long linear carbon chains known to be abundant in interstellar space."... 

There has been particular interest in the chemistry of small organosilicon ions and their neutral counterparts for a number of reasons (i) Small SiCvHv molecules are well suited to draw analogies between the structural chemistry of silicon and carbon—or to state differences, (ii) The chemistry of small silicon compounds is viewed as fundamental in astrophysics and astrochemistry, and a large number of cationic and neutral SiRv molecules have been detected in interstellar and circumstellar matter. 

A significant fraction of the fullerenes and buckyonions in the interstellar medium could be hydrogenated as discussed by Webster in 1992 (see Fig. 1.4). These molecules, named genetically as fulleranes have deserved attention as potential carriers of diffuse intestellar bands and other interstellar and circumstellar features (Webster 1991, 1992, 1993a). Both, fullerenes and fulleranes have been detected in samples of the Allende meteorite (Becker et al. 1994), see Fig. 1.3b. 

Many carbon rich stars also present an important emission at 11.3 pm associated with solid carbon and some of them present nebulosity of reflection as a consequence of the scattering of the circumstellar grains. There are indications that in the material ejected by these stars, carbon must exist, apart from CO molecules and solid grains, in some other form or species until now unknown, fullerenes are a possibility. Unfortunately, there is very little information about the presence of molecules of intermediate size (between 10 and 106 atoms) in circumstellar regions. There are bands in carbon rich planetary nebulae, for example those of 3.3,6.2,7.7, 8.6 and 11.3 pm which have not been detected in carbon stars but are observable in transition objects evolving between the giant red phase and the planetary nebula as for example, the Egg Nebula (Fig. 1.5) and the Red Rectangle. These infrared bands are normally associated with the vibration modes of materials based on carbon, possibly PAHs. But until now it has not been possible to make a conclusive identification of the carrier. 

Hydrides of the fullerene C60 have also been investigated as potential DIB carriers (Webster 1992, 1993b). No specific identification has been suggested but it should be noted that the conjugated systems of -electrons are predicted to have transitions in the visible range. The predicted optical and near-infrared transitions of fullerene based molecules may offer a potential explanation for the long-standing problem of the diffuse interstellar bands and other interstellar and circumstellar features. 

Petrie and Bohme (2000) and Millar (1992) have studied the interstellar fullerene chemistry focusing on ion/molecule chemistry in various astrophysical environments. Fullerene based molecules could play a relevant role in the chemistry and physics of the interstellar medium and circumstellar environments (see also Watson et al. 2005 Webster 1991 Cataldo 2003). 

Infrared and radio observations of the circumstellar envelope have been exploited very successfully. The most commonly studied molecule is CO hundreds of nearby stars are known CO sources (Nyman et al. 1992). Additionally, over 50 different species have now been detected in various circumstellar envelopes (Olofsson 1992). 

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