Hubble space telescope observations of proto-planetary nebulae

In this thesis, I present a high angular resolution imaging study on nine proto-planetary nebulae (PPNs) with the Hubble Space Telescope. Seven of the PPNs show prominent biopolar nebulosity with various complexities such as point-symmetric structures and collimated outflows, suggesting that the dust shells in PPNs possess significant departures from spherical symmetry which is usually found in the dust shells in the preceding Asymptotic Giant Branch (AGB) phase.; Assuming an axi-symmetric density distribution in the circumstellar dust shell, our numerical model (NONSP) has successfully simulated the bipolar morphology, searchlight beams and dark lane structures in the observed optical images as well as obtained satisfactory spectral energy distribution fittings. Our NONSP model has shown that an object's orientation on the sky and the degrees of asymmetry in its circumstellar dust shell have a great impact on the optical morphology and spectral energy distribution.; Deep, high-resolution imaging has helped to detect circumstellar are structures in an increasing number of PPNs (a total of 5 objects and 4 in this thesis). The presence of these circular arcs in PPNs with a variety of viewing orientations implies that the arcs are illuminated portions of spherical shells, viewed in scattered light. The arcs are likely formed by density enhancements of the mass loss during the preceding AGB phase. The separations of the arcs translate to time scales of a few hundred years, which cannot be explained by the stellar pulsational period (∼1 yr) or the nuclear thermal pulse period (∼104 yr).; A collimated bipolar outflow emerging from a visible disk around the proto-planetary nebula IRAS 17106-3046 is discovered. The radial intensity profile of the disk suggests that it is consistent with an expanding torus. A recently formed jet breaking out of the bipolar lobe is also seen in the system.; Our observational and theoretical results suggest that the asymmetry needed to explain the origin of asymmetric planetary nebulae (PNs) is already present in PPNs. Therefore, the asymmetry in circumstellar dust shells must have been developed quickly in the very late AGB phase or the early PPN phase.