Carbon-rich stars

The synthesis of PAHs is generally thought not to occur within the clouds themselves, but in the envelopes of old carbon-rich stars. These stars possess large envelopes in which material expands outward and cools as it expands. In the inner... 

What is the ultimate fate of the molecular material formed in the envelopes of carbon-rich stars as it heads out into space The dust grains will be processed only slowly by the interstellar radiation held and survive almost intact until they become part of an interstellar cloud. The survival of individual PAHs depends on their size the larger ones withstand radiation much better than do the smaller ones.115 By survival we are referring to the aromatic skeleton the interstellar radiation field will efficiently break H bonds and cause ionization so that unsaturated, ionized PAHs are likely to dominate those found in the diffuse interstellar medium. Such species have been suggested as a source of the DIBs.118,123 Small molecules photodissociate in the interstellar radiation field before the material becomes part of an interstellar cloud. 

It is important to note that we have tried to avoid carbon-rich stars, because they have a rich molecular line spectrum, mostly CN, CH and C2, obliterating many interesting atomic lines of rare elements. This is why we had in our sample a star, CS 31082-001, in which we were able to measure the 385.97 nm line of U II, whereas in the similar r-process element enriched star CS 22892-052, but carbon rich, a CN line obliterates the U II line. 

Our observation has confirmed the oxygen-rich chemistry of the circumstellar gas around the carbon-rich star V778 Cyg. The radial vleocity of the maser peak (-17 km s 1) is red-shifted by only 2 km s 1 relative to the optical photospheric velocity (-19 km s 1)- Therefore, the expansion velocity of the maser emission career in V778 Cyg seems 2 to 10 km s 1, a moderate value contrary to the case of EU And. 

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. 

The carbon-bearing material in the outflows from luminous carbon-rich stars... 

The carbon-rich stars losing a large amount of mass are a fascinating laboratory for studying carbon chemistry in space. Below, I describe what is known about these stars, and how we can hope to use them to understand the nature of celestial carbon-chemistry. 

While we have direct observational evidence for small molecules and large grains, we have very little information about clusters in the outflows from carbon-rich stars. That is, we know very little about the particles with more than 10 atoms, but, say, fewer than 10 atoms in circumstellar regions (see Kroto Jura 1992). 

Silicon carbide is thought to be an important component of the dust shells surrounding carbon-rich stars it is likely, therefore, that astrophysical studies of the SiC radical will be significant in the future. 

Pascoli G. Polleux, A. Condensation and growth of hydrogenated carbon clusters in carbon-rich stars. Astron. Astrophys. 2000, 359, 799-810. 

How can a scientist remain indifferent in light of the fact that carbon chains have been detected by radio astronomers in the molecular clouds present in the interstellar medium or in the circumstellar medium of carbon-rich stars or in the atmosphere of certain bodies of the Solar System such as Titan, Saturn s giant moon The wonder increases further when it is realized that about one thousand organic molecules classified as polyynes are produced by plants, fungi, and microorganisms and play a biological role in the biosphere and may be used in the treatment of diseases as antibiotics, anticancer or, more simply, as anti-infective agents. 

There are other classes of carbon-rich stars including the cool and warm R-type stars, the 13C-rich J-type stars, CH-stars, and dwarf carbon stars [73]. Barium stars also show enrichments in carbon and heavy elements, although they have C/O < 1 in general. Possibly 20% of all very metal-poor with [Fe/H] < —2 are also carbon rich, with [C/Fe] 2 in some cases [20]. Stars in these other classes are not on the AGB and are not responsible for producing their own carbon enrichments. Some of them, such as the barium and CH-type stars, are all known binaries [87,88] and thus presumably obtained their carbon from a former AGB companion. These stars are also devoid of Tc enrichments [89,90]. The warm R-type stars are all single stars [91], and may all result from some type of binary-star merger event (see Izzard, Jeffery Lattanzio [92] and references therein). The rare J-type stars, with very low 12C/13C ratios, are still a mystery [93,94]. 

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... 

It has been speculated that buckyballs might occur in outer space around carbon-rich stars, and, rather surprisingly, they may even exist at the cores of mundane soot particles, which tend to be spherical. In recent years, fullerenes of C70 and C50 have been proposed as stable even clusters of carbon atoms. 

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