Evolved stars and planetary nebulae are rich and varied sites of molecule and dust formation. These objects undergo dramatic mass loss which ultimately enriches the interstellar medium. In this thesis, a number of studies, outlined below, have been undertaken to better understand the chemical and physical properties of these diverse objects. A molecular line survey of a sample of evolved stars and planetary nebulae has been carried out using the Mopra radio telescope, Australia. Transitions with hyperfine structure have been fitted to constrain optical depths. The population diagram method was applied to determine the rotation temperatures of molecules which had multiple transitions available. Column densities have been calculated for all detected species and isotopic ratios measured where possible. The results include the corroboration of the classification of II Lup as a J-type star. The 89.087 GHz HCN maser was detected in IRAS 15082-4808 for the first time from the aforementioned survey, bringing the total number of detections of this maser to ten. The velocity shift of this maser has been measured at −2.0+/-0.9 km/s. Drawing on literature data in addition to the survey data, the variation of maser intensity with pulsation phase has been investigated across all sources for the first time. Comparing these masers with model atmospheres constrains the formation region to between 2 and 4 stellar radii. CO in the circumstellar envelope of II Lup has been modelled using the radiative transfer codes GASTRoNOoM, and ComboCode. The models have demonstrated that a ‘standard’ smooth model does not satisfactorily reproduce the combined CO observations of PACS, JCMT, Mopra and APEX. Two potential solutions are proposed: a discontinuous temperature model, requiring the presence of an efficient cooling molecule that is most effective in the region 75-200 R*, or a variable mass loss model that requires a factor of ten increase inmass loss in the same region. Zinc abundances, a proxy for iron abundances, have been determined for a sample of Galactic planetary nebulae using the [Zn IV] 3.625 μm line. O++/O has been shown to be a reliable ionisation correction factor for Zn3+ from Cloudy photoionisation models. The majority of the sample are sub-solar in [Zn/H] and enriched in [O/Zn]. Zinc abundances as functions of Galactocentric distance have also been investigated and no evidence for a trend has been found.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:632289 |
| Date | January 2014 |
| Creators | Smith, Christina Louise |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://www.research.manchester.ac.uk/portal/en/theses/spectroscopic-studies-of-evolved-stars-and-planetary-nebulae(c01465c8-a7dd-4471-a37b-cc4ee01a9905).html |
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