Circumstellar chemistry

In this contribution, we first examine the known mass-loss mechanisms in the light of the recent observations(Sect.II),and then we review some recent infrared and radio observations that may be relevant to our understanding of the outer atmospheric structure and mass-loss phenomena(Sect.Ill). Based on these observations, new picture of the outer atmosphere of red (super)giant stars is proposed and its implications on circumstellar chemistry as well as on mass-loss phenomena are discussed(Sect.IV). 

The circumstellar chemistry is often subdivided into three main zones, which are determined by a comparison of the characteristic dynamic flow time, R/vx, with the chemical reaction times (Lafont et al. 1982 Omont 1987 Millar 1988). (i) In the region closest to the star (perhaps R 1014 cm), the density is sufficiently high that three-body chemical reactions occur in a time short compared to the dynamic time. In this regime, we expect the chemical abundances to approach thermodynamic equilibrium, (ii) Somewhat further away from the star (1014 cm < R < 1016 cm), there is a freeze-out of the products of the three-body reactions (McCabe et al. 1979). In this region, two-body reactions dominate the active chemistry, (iii) Finally, far from the star (R > 1016 cm), the density becomes sufficiently low that the only significant chemical processing is the photodestruction that results from absorption of ambient interstellar ultraviolet photons by the resulting molecules that flow from the central star. 

Circumstellar Chemistry and Dust from Dead Stars... 

Much work on circumstellar chemistry remains to be done. Mamon et al. (1988) have shown that self-shielding and mutual shielding play an important role in determining the spatial distribution of CO and hence of Cl and C in CSEs. The details of this process need to be included in a comprehensive chemical model. The recent detection of several sulphur species in IRC fl0216 (Cernicharo et al. 1987) and in OH231.84-4.2 (Morris et al. 1987) should encourage the inclusion of sulphur chemistry in models. This may provide some insight into models of interstellar sulphur chemistry for which much uncertainty remains. 

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. 

Circumstellar medium The environment around a star that has chemistry driven by the photon flux or radiation field. 

Prinn, R. G. (1993) Chemistry and evolution of gaseous circumstellar disks. In Protostars and Planets III, eds. Levy, E. H. and Lunine, J. I. Tucson University of Arizona Press, pp. 1005-1028. 

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). 

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. 

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. 

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). 

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. 

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. 

This chemistry occurs in the transition zone (Figure 1), where stellar winds drive hot gas from the photosphere into the circumstellar environment. Dust... 

Stars with initial masses of -1-8 M evolve into red giant stars and lose their outer atmospheres through stellar winds. The lost material creates huge circumstellar shells. The overall composition of a CSE is determined by the ongoing nucleosynthesis in the star. Most importantly, production and dredge-up of C in AGB stars changes the surface composition from oxygen rich (C/0<1 in M stars) to carbon-rich (C/0>1 in C stars). The C/0 ratio determines the gas chemistry in the CSE and which condensates (e.g., silicates or carbides) appear. 

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