Circumstellar Ejecta Surrounding Evolved, Intermediate Mass Stars

In an attempt to catalogue and understand the outflows from evolved, intermediate-mass stars, three projects concerning stellar nebulae are presented. The first of these is a new method for determining distances to planetary nebulae (PNe) known as the expansion parallax algorithm. The ionized shells of 18 PNe were observed at the Very Large Array in the lambda6 cm continuum during two epochs separated by several (~ 5) years. The data from both epochs were subtracted in the Fourier plane and analyzed to determine the angular expansion rate of the nebulae, which is often very small( ~1 mas/yr). By combining this result with the Doppler expansion rate, the distance to a PN can be computed. Though other methods failed to provide distances to PNe that are smaller than the uncertainties, the results of the expansion parallax method yield errors of ~10-30% for several targets. The ionized gas discussed above was once neutral, when the protoplanetary nebula (PPN) phase reigned. Since their lifetimes are short and their central stars are too obscured to be visible optically, PPNe are rare astronomical objects. The circumstellar shroud surrounding PPNe is neutral in character, and thick enough to support molecules. Defying observations at optical and shorter wavelengths due to the high dust density in the nebula, and lacking any radio emission except from a tiny expanding HII region, PPNe are best observed with a millimeter interferometer tuned to any of the copious molecular emission lines emanating from the nebula. For the second project, the protypical PPN CRL 618, has been imaged with the Owens Valley Radio Interferometer in the 98 GHz (3.1 mm) CS(2-1) transition and in the 115 GHz (2.6 mm) CO(1-0) transition. The CS transition is sensitive to high density material, and the images reveal the marginally resolved core of CRL 618. The more diffuse halo of CRL 618 is probed by the CO gas and exhibits previously undetected structure. The final project described herein involves Fast, Low Ionization Emission Regions (FLIERs), which are localized regions of low ionization atomic gas. FLIERs are found in pairs located along the major axis of several PNe. Because they bear anomalous kinematic and chemical signatures, and because there is no known method of collimation and ejection from a PN nucleus, FLIERs are enigmatic objects. Observations are presented of the FLIERs and similar low ionization microstructures along the major axis of 5 PNe obtained with the Palomar 5-m telescope equipped with the Double Spectrograph. Emission lines in the medium resolution spectra were analyzed with a 5 level model atom to reveal nebular diagnostics and chemical abundances in the emitting gas. Results are presented for several conspicuous positions along the nebular major axis for all targets.