Circumstellar envelopes processing

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). 

Astrochemistry The theoretical study of chemical processes in cosmic environments and the observational determination of physical parameters through the study of abundances of molecular species. This review concentrates on the recent results concerning circumstellar envelopes and the interstellar medium. The field deals, however, with synthesis of molecules in cometary nuclei and planetary atmospheres, as well as steUar photospheres. 

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. 

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. 

After about one million years (for solar-mass stars, this process is much faster for higher masses), the combination of outflow and infall disperses the majority of the envelope and the star is optically revealed, although a circumstellar disk is still present. For solar-mass stars, this is the T Tauri phase, while for intermediate masses, these stars are referred to as Herbig Ae/Be stars (Hillenbrand et al. 1992). Several million years after the primordial disk has almost disappeared. 

---