Circumstellar Distribution of Dust Grains Around Proto -Planetary Nebulae: Observational Evidence for Axisymmetry and 2.5-D Radiative Transfer Model Calculations

Proto-planetary nebulae (PPNs) are evolved, intermediate initial mass (0.8--8 solar masses) stars that are in rapid transition from the asymptotic giant branch (AGB) phase into the planetary nebula (PN) phase. Largely spherically symmetric circumstellar dust shells of AGB stars are known to evolve into mostly axisymmetric PNs. Since the PPN phase predates the period of final PN shaping due to interacting stellar winds, the PPN shells preserve pristine records of AGB mass loss histories. Hence, the origin of the structure formation in the circumstellar shells is investigated by probing the density distribution in the PPN shells. First, presented are the results from imaging surveys of PPNs at the mid-infrared and optical. In these imaging surveys, a prevalent axisymmetry has been found to reveal the equatorially-enhanced (toroidal) structure of the PPN shells. Moreover, two morphological types (SOLE-toroidal and DUPLEX-core/elliptical PPNs) have been discovered among PPNs in each survey, which strongly suggest the optical depth of the PPN shells is an important determining factor of the PPN shell morphology. The combined results of the two surveys have indicated that the PPN shells are intrinsically axisymmetric due to equatorially-enhanced superwind mass loss occurring near the end of the AGB phase, and a variable degree of equatorial enhancement in the PPN shells yields distinct optical depths of the shells, which determine the shell morphologies. Second, we present a newly developed multi-dimensional radiative transfer code, 2- D D UST , and its results to derive physical parameters concerning the characteristics of the PPN dust shells without ambiguities arising from the inclination angle effect. Model calculations with a single dust density distribution function have reproduced all the observed morphologies quite well. Numerical analyses have suggested that the PPN shells are indeed highly equatorially-enhanced due to an axisymmetric superwind, and SOLE-toroidal PPNs originate from lower mass progenitor stars and become elliptical PNs while DUPLEX-core/elliptical PPNs arise from higher mass progenitor stars and evolve into bipolar PNs. These imaging and numerical analyses have yielded well-constrained models that need to be explained by future dynamical models.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Astronomy

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2015

Author dc:creator

• Ueta, Toshiya

Contributors dc:contributor

• Margaret Meixner

• Physics, Astronomy and Astrophysics

Language dc:language

eng

Identifier

(MiAaPQ)AAI3044247

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/85171