Circumstellar shells

Two emission spectra of circumstellar shells are shown in Fig. 14.5, for an oxygen star in the bottom part of the figure and for a carbon star in the top. 

Infrared spectra of evolved stars are generally dominated by the radiation from their circumstellar shells. M stars are characterized by the 10 pm emission feature from silicate dust grains, while C stars by the 11 pm SiC band. However, some C stars have been found to show the 10 pm feature indicating the oxygen-rich property of their circumstellar dust (Willems and de Jong 1986, Little-Marenin 1986). 

In 1985. similar experiments were conducted at Rice University. In a 1988 paper. Curl and Smalley (Rice University) outlined their experiments with carbon cluster beams, essentially using the clusler-generaung apparatus previously described by the Exxon researchers. Initially, this experimentation was motivated by an interest that had been shown by die astrophysicist, Krotu (University of Sussex), who had been modeling the formation of carbon molecules in circumstellar shells. As a consequence, the Rice University team concentrated its studies on the smaller (2- to 30-atom) carbon clusters. As pointed out in the Curl-Smalley paper, the objective was to determine if some or all of the species had the same form as the long linear carbon chains known to be abundant in interstellar space."... 

We can estimate the column density of material in a circumstellar shell, NH, extending from radius R to infinite distance, from the expression ... 

Linear hydrocarbon radicals have been the subject of intensive laboratory spectroscopic and radio-astronomical research since the early 1980s. In recent years, a considerable number of rotational spectroscopic studies of medium to longer hydrocarbon chains such as C5H, CeH, CgH, and ChH have been carried out using a pulsed molecular beam FTMW spectrometer. The high resolution offered by such a spectrometer allowed the detection of the hyperfine sphtting of rotational transitions. These measurements improved fine and hyperfine coupling constants and provided rest frequencies with accuracies better than 0.30 km s in equivalent radial velocity up to 50 GHz. Indeed, some of the small C H radicals with n < 9 have subsequently been detected in space, in molecular cloud cores, and in certain circumstellar shells. These hydrocarbon chains are among the most abundant reactive space molecules known. 

Little-Marenin I. R. (1986) Carbon stars with silicate dust in their circumstellar shells. Astrophys. J. 307, L15-L19. 

Assuming that our somewhat speculative structure is correct, there are a number of important ramifications arising from the existence of such a species. Because of its stability when formed under the most violent conditions, it may be widely distributed in the Universe. For example, it may be a major constituent of circumstellar shells with high carbon content. It is a feasible constituent of interstellar dust and a possible major site for... 

Kroto HW, Heat JR, O Brien SC, Curl RF, Smalley RE. Long carbon chain molecules in circumstellar shells. Astrophys. J. 1987, 314, 352. 

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. 

However, as temperatures and densities drop steeply in the circumstellar shell with increasing distance from the central star, reaction kinetics may no longer permit establishment of thermochemical equilibrium in the stellar outflow 4). Detailed and generally applicable discussions of the reaction kinetics relevant here can be found in 52, 53). Let us follow a parcel of hot atmospheric gas traveling away from the photosphere and assume that a certain gas is produced or destroyed by some reaction in the CSE. As long as chemical... 

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. 

Figure 8. The end product of the effort a transition of MgCN, measured in the lab, is found by a radio telescope in the circumstellar shell of the evolved star IRC- 10216. The doublet structure, due to an unpaired electron in this species, is clearly visible in both spectra.

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