An important area of current astronomical research concerns the study of mass loss of red giant stars as they evolve to white dwarfs through the "planetary nebula" stage. Although this research is significant to an understanding of stellar evolution in its own right, it is also important because planetary nebulae are bright and may be observed comparatively easily. Thus, planetary nebulae are particularly convenient objects to study to learn the properties of "circumstellar matter", material which has has been recently ejected from hot stars in densities intermediate between stellar and interstellar gas material. Matter in a similar state also occurs around stars for different reasons, in particular as "disks" around interacting binary stars, B-emission stars, and dense molecular interstellar clouds. However, in these environments it is often difficult to observe this gas and to diagnose its properties. The Principal Investigator (PI) proposes to continue previous NSF-supported research on the physical and chemical properties of matter as it leaves the star in a dense "wind" and eventually mingles with ambient interstellar gas. The PI will determine reaction rates among atoms, molecules, and radiation (starlight) in order to understand the distribution of forms of matter (ions, neutral atoms, and molecules). Because these atoms radiate away thermal energy that they absorb from gas motions, this knowledge will lead to an understanding of the temperature and density of matter at different distances from the star. Comparisons of these theoretical predictions can then be made with new observations from radio (millimeter wavelength) arrays. The correctness of the models will then be evaluated.
