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Astron. Astrophys. 348, L17-L20 (1999)

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5. Conclusions

Dynamical models for the outer atmospheres for oxygen-rich AGB stars reveal a layered structure of the molecules and much higher column densities than predicted by hydrostatic models. This is a natural consequence of the shock waves occurring in these models, even in case of a low pulsational activity. The overall amount of levitated gas found in the model exceeds the lower limits inferred from observations. Therefore, the levitation by shock waves seems to provide a physical explanation for the generation of "warm molecular layers" as suggested by recent ISO observations, though it is not yet clear, whether this effect alone can explain the very large amounts of observed molecules. Other effects like dust formation and chemical non-equilibrium (in particular concerning SO2 and CO2) are probably involved.

The rotational excitation temperatures of CO, CO2, and SO2 are found to remain close to the kinetic gas temperature down to densities of [FORMULA]. Therefore, the observed rotational fine-structures of the vibrational bands of these molecules provide a reliable and easy-to-access probe for the kinetic gas temperature in the outer atmospheres of AGB stars. However, optical depths effects, velocity gradients, and vibrational non-LTE effects should be taken into account.

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© European Southern Observatory (ESO) 1999

Online publication: July 16, 1999