RUI: High-Resolution Observational Study of Proto-Planetary Nebulae

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Abstract Hrivnak AST 99-00846The observational properties of intermediate and low mass stars have been well documented in most phases of their evolution, from the main sequence to the white dwarf phase. The most poorly observed phase of evolution, however, is the transition from red giants to planetary nebulae, in what is known as the proto-planetary nebulae (PPN) phase. This is because the transition occurs relatively rapidly; the stars are often significantly obscured by circumstellar dust; and because until recently there was not efficient means to identify such objects. In the past decade, however, as a result of observations obtained from the IRAS satellite infrared sky survey, about 50 good PPN candidates have been identified based upon the infrared emission from their dust shells.The goal of this project is to determine quantitatively the observational properties of the central stars and circumstellar shells of a representative number of objects in this transitional phase. High-resolution spectroscopic observations will be obtained at visible and near-infrared wavelengths. From the data of these observations, the photospheric abundances and the chemistry and dynamics of the circumstellar envelopes will be determined. The results will help test theories of nucleosynthesis and mass loss in low- and medium-mass stars. It has been found that most proto-planetary nebulae vary in brightness and also pulsate. The pulsation velocity is measured by observing the Doppler effects in the observed emissions from the stars. This unique program to monitor this variability of the proto-planetary nebulae will be carried out over the next three years. In principle, the results obtained may lead to information on the internal structure of these objects