Evolved stars, circumstellar envelopes

Abstract A molecular line search in the range between 85 and 89 GHz has been performed in the circumstellar envelopes of 11 evolved stars. Emissions of 29SiO J=2-l,28SiO J=2-l, HCN J=l-0, H13CN J=l-0, HC5 N J=33-32, HCO+ J=l-0 transitions and other transitions of C2 H, C4 H, and C3 N have been observed in 11 stars. We have detected the ground state 29SiO J=2-l maser in several stars. We have also detected HCN emission in VY CMa. A narrow H13CN spike feature near the central velocity has been found in the spectrum of CRL 2688. 

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. 

Plate 2, The circumstellar envelope of the evolved star CRL 2688, showing outflowing material (light blue HST image) and resulting molecular shell (contours), (See page 8 of color inserts.)... 

Radio astronomical spectra trace more than just the chemical content of a given gas cloud. They also indicate the physical conditions and kinematic motions in a source. Rotational transitions arising from levels that are high in energy trace gas that is very hot, for example. Very narrow lines indicate cold, quiescent gas. Objects with gaseous outflows, such as circumstellar envelopes, have broader spectra. If the dispersing material comes from a hollow shell, such as in the case of the evolved star IRC+10216, the molecular lines can have a U-shape, as shown in Figure 8. The spectral shape may not look at all like what has been measured in the laboratory however, it is essential that the center... 

A number of molecules have been detected in the interstellar medium, in circumstellar envelopes around evolved stars, and comae and tails of comets through observation of their microwave, infrared, or optical spectra. The following list gives the molecules and the particular isotopic species that have been reported so far. Molecules are listed by molecular formula in the Hill order. All species not footnoted otherwise are observed in interstellar clouds, while some are also found in comets and circumstellar clouds. The list was last updated in October 2008 and lists 162 molecules (298 isotopic forms). 

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. 

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