Circumstellar chemistry shells

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. 

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. 

At temperatures and densities of circumstellar shells, the chemistry is in general likely to be highly non-equilibrium. However, in the hotter and denser Inner parts of stellar winds, oiroumstellar shells and in particular cool stellar atmospheres, equilibrium chemistry may be a reasonable approximation, and can at least give us a basis from which non-equillbrium chemistry could be considered. 

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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