Circumstellar envelope chemistry

In order to investigate the gas phase chemistry of the circumstellar envelopes around these peculiar objects, we have observed radio molecular lines of H20, SiO, HCN, and CO towards three of them BM Gem (C5, 4J), V778 Cyg (C4, 5J), and EU And (C4, 4). 

In contrast to the dark cloud chemistry, the molecules in circumstellar envelopes (IRC -f 10216) seem to be created continuously in a small, high temperature high density layer- which allow fast thermodynamic equilibrium- and subsequently expelled into the lower density cool envelope. There they are observed with an... 

In this chapter, we reviewed the methods and results of chemical equilibrium calculations applied to solar composition material. These types of calculations are applicable to chemistry in a variety of astronomical environments including the atmospheres and circumstellar envelopes of cool stars, the solar nebula and protoplanetary accretion disks around other stars, planetary atmospheres, and the atmospheres of brown dwarfs. The results of chemical equilibrium calculations have guided studies of elemental abundances in meteorites and presolar grains and as a result have helped to refine nucleosynthetic models of element formation in stars. 

This chapter briefly introduces the chemistry in circumstellar envelopes (CSE) around old, mass-losing stars. The focus is on stars with initial masses of one to eight solar masses that evolve into red giant stars with a few hundred times the solar radius, and which develop circumstellar shells several hundred times their stellar radii. The chemistry in the innermost circumstellar shell adjacent to the photosphere is dominated by thermochemistry, whereas photochemistry driven by interstellar UV radiation dominates in the outer shell. The conditions in the CSE allow mineral condensation within a few stellar radii, and these grains are important sources of interstellar dust. Micron-sized dust grains that formed in the CSE of red giant stars have been isolated from certain meteorites and their elemental and isotopic chemistry provides detailed insights into nucleosynthesis processes and dust formation conditions of their parent stars, which died before the solar system was bom 4.56 Ga ago. 

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... 

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). 

Most, if not all, astrophysical objects are not static over a period of time long enough for the chemistry to reach steady state, except perhaps in the diffuse medium as long as the initial H2/H abundance ratio is assumed to be non-zero. As a consequence, the modifications of the physical conditions with time have to be considered. A good example is the modelling of circumstellar envelopes. The cells of material pushed away from the star encounter lower temperatures and densities. Close to the star, the temperatures are so high that species are only in atomic form. As the material moves away from the star, molecules wiU be formed and survive until they encounter a much thinner medium where the interstellar UV field will destroy them. A schematic view of the physical structure, which is far more complex than discussed here, and the chemical composition of a circumstellar envelope are given by Fig. 4.3. 

In molecular clouds, atomic species with ionization energies greater than 13.6 eV must be predominantly neutral because of the shielding effects of neutral hydrogen. It is mainly the heavier elements, such as C, N, and O, which are observed in the peripheral portions of the clouds to be in the partially ionized state. For circumstellar envelopes, cosmic rays lose out to photo processes and the chemistry is mediated by the input of stellar photospheric radiation (in the hotter stars and in novae and supernovae) and from the diffuse interstellar radiation field. 

Many complex polycyanoacetylenes are observed in both circumstellar envelopes and dense molecular clouds, the heaviest being HCnN, one of the cyanopolyynes. As mentioned in the section on carbon chemistry, these... 

The morphology and extent of circumstellar envelopes is determined by the dynamics of the mass loss process and by the stellar and interstellar radiation field. Since the envelope s density structure is approximately known, the time exposure of the interstellar radiation field can be estimated, and the distribution of molecular photoproducts can be measured. Stellar envelopes represent an unparalleled astrochemistry and radiative transfer laboratory. For this reason, evolved stars have been prime targets for the detection and study of rare molecules in the overall investigation of interstellar chemistry. 

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. 

We have shown that with the assumption of equilibrium chemistry, the condensation of grains In cool stellar atmospheres and circumstellar shells can In certain cases substantially modify the chemistry of the gas phase. For the envelopes with positive detection of HCN. Deguchi et al. (1986) estimated the circumstellar abundance relative to H2 at 10 to 10. Reference to figure 1 indicates that for a C/O ratio close to solar only a minute Increase of a very small HCN abundance can occur, but tor C/O > 0.82. much larger enhancements in the abundance can take place, becoming comparable to the observed values. 

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