Evolved stars including asymptotic giant branch stars, proto-planetary nebulae and planetary nebulae are well known to be sources of molecules for the interstellar medium. Circumstellar envelopes of asymptotic giant branch stars are known to be sites of active molecular synthesis. These molecular species are then distributed to the interstellar medium as the envelope is gradually ejected in the proto-planetary nebula and planetary nebula phases. As carbon that is dredged-up from the stellar core by convectional instabilities facilitates rapid formation and reprocessing of molecules, the chemical abundances differ significantly between objects at different stages of stellar evolution. Changes in the physical conditions in the envelope also allow different chemical processes to take place throughout the evolutionary sequence. Conducting spectral line surveys on a large sample of objects at different stages of stellar evolution will allow one to track those chemical and physical changes along the evolutionary track.
Presented in this thesis are the results from a series of unbiased systematic line surveys targeting the circumstellar envelopes of evolved stars. As part of an on-going effort, these surveys aim to expand the coverage both in terms of objects at different evolutionary stages and wavelength ranges in order to paint a more complete picture in our understanding of the chemical evolution in objects at the final stages of stellar evolution. All surveys included in the present study are conducted using the 45m radio telescope at the Nobeyama Radio Observatory, and the same instrumental settings were used when observing a given frequency region to minimize the systematic uncertainties. The four objects surveyed in this study, the asymptotic giant branch star CIT6, the proto-planetary nebulae CRL 2688 and CRL 618, and the planetary nebula NGC7027, represent a healthy sample from all key phases of late-stage stellar evolution. Since the surveys are conducted in millimeter-wavelengths, they are mostly targeting rotational transitions from molecules. In particular, rotational transitions associated with the cyanopolyyne chain HC7N reported in the survey of CIT6 and CRL 2688 are novel detections.
Spectra from the series of surveys are analyzed using previously obtained spectra of the prototypical asymptotic giant branch star IRC+10216 as a benchmark. Results from fractional abundance computations suggested that CIT6 is a more evolved specimen relative to IRC+10216 and supported chemical models pertaining to the synthesis of cyanopolyyne chains. Intensity ratios in CRL 2688 showed that the cyanopolyyne enrichment towards conclusion of the asymptotic giant branch phase is no longer present in proto-planetary nebulae. Rotational analysis provided evidence to support the hypothesis that the ammonia absorption in CRL 618 originates from two distinct components. Flux ratios in NGC7027 also showed that the recombination lines in the planetary nebula behave according to theoretical predictions. Altogether, these findings will complement previous results and enable one to trace and understand the interplay between chemistry and physical conditions within the circumstellar envelope as objects proceeds on the evolutionary track. / published_or_final_version / Physics / Master / Master of Philosophy
| Identifer | oai:union.ndltd.org:HKU/oai:hub.hku.hk:10722/193464 |
| Date | January 2013 |
| Creators | Chau, Wayne C, 周穎鏘 |
| Contributors | Kwok, S, Zhang, Y |
| Publisher | The University of Hong Kong (Pokfulam, Hong Kong) |
| Source Sets | Hong Kong University Theses |
| Language | English |
| Detected Language | English |
| Type | PG_Thesis |
| Rights | Creative Commons: Attribution 3.0 Hong Kong License, The author retains all proprietary rights, (such as patent rights) and the right to use in future works. |
| Relation | HKU Theses Online (HKUTO) |
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