Circumstellar dust shells

Stars further along in their life cycle are often cooler and redder than younger stars. Shells of dust that has been condensed from material ejected from these cool stars often surround them. Such circumstellar dust shells, heated by the stars, emit strongly in the infrared with a spectrum characteristic of absorption bands in the dust the emissivity of a small particle is equal to its absorption efficiency (see Section 4.7). An excellent review of circumstellar dust has been given by Ney (1977). 

The R CrB variables and related hydrogen-deficient carbon stars show strong carbon features (except CH) and very weak Balmer lines. The R CrB stars are surrounded by circumstellar dust shells, and continue to eject puffs of new circumstellar material on time scales of a month or two mass loss rates of order 10- Mo yr- are reported by Walker 1986. The compositions of the R CrB stars and hydrogen-deficient carbon stars are were compiled by Lambert (1986). Hydrogen is extremely deficient in both groups (H/He 10 J-10 °), and C/Fe is enriched by typically an order of magnitude over the solar ratio. C/0 is typically 2 (more than is measured in the AGB stars, and [N/Fe] 1. R CrB itself contains a strong lithium... 

The surface distribution of M stars is studied by differentiating them according to whether they show a circumstellar dust shell (CS) or not. Analysis shows that galactic latitudinal and longitudinal distributions are not determined by spectral subclasses alone. The study also indicates that the M type stars with CS have higher intrinsic luminosities in the K band than those without CS. The M stars used in the study are obtained from the Two Micron Sky Survey catalogue (IRC) which is an unbiased sample with respect to the interstellar extinction. The CS feature is identified by the ratio of flux densities at 12 and 25 m in the IRAS point source catalog. 

With improved possibilities for infrared spectroscopy, broad extinction bands around 9.7 pm and 18 pm have been detected, which were ascribed to the stretching (Woolf Ney 1969) and bending (Treffers Cohen 1974) modes in the SiC>4 tetrahedron forming the building block of silicates, because they correspond to known absorption bands seen in all terrestrial silicates. These bands are also seen in the emission from dust shells around O-rich stars. This gave the first observational hints on the mineralogy of the silicate dust. The smooth, structureless nature of the bands indicated that the silicates in the ISM and in circumstellar dust shells are amorphous. 

Since jqZq seems to set the lower limit for efficient dust formation,4 planetary systems are only expected to form around stars with at least this metallicity. The relative abundances of rock-forming elements in such systems are essentially the same as in the Solar System. Only the total amount of such elements may vary considerably, depending on the birthplace and birth-time of such systems. Planetary systems with unusual elemental compositions, e.g. like those observed in many highly evolved stars, are not expected to exist in particular, oxygen is always more abundant than carbon such that for planetary systems there is no counterpart to the carbon-rich circumstellar dust shells. 

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). 

An adequate answer to these questions must be based on the detailed study of the processes of formation and growth of dust particles in these environments. However, dust formation cannot be considered as an isolated problem because due to their huge absorption cross sections even a small contamination of the atmospheres by circumstellar dust may have a significant influence on the radiative transfer and (via energy- and momentum-coupling) on the thermodynamic and hydrodynamic structure of the dust forming shell. 

Carbonates are common in hydrous meteorites and hydrous IDPs, where they are believed to have formed by parent-body aqueous processing. Since simple models of cometary evolution involve no aqueous processing, carbonates were generally presumed not to occur in comets. However, carbonates have also been detected by infrared spectroscopy in the dust shell around evolved stars and in protostars, where liquid water is not expected (Ceccarelli et al. 2002 Kemper et al. 2002). Indeed, Toppani et al. (2005) have performed experiments that indicate that carbonates can be formed by non-equilibrium condensation in circumstellar environments where water is present as vapor, not as liquid. Detections of carbonates in other exosolar systems are reported by Ceccarelli et al. (2002) and Chiavassa et al. (2005). 

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... 

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. 

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