Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492341
Title: Studies of the chemical composition of B-type post AGB stars
Author: Thompson, H. M. A.
Awarding Body: Queens University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2008
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Abstract:
This thesis focuses on the analysis of two UV-bright globular cluster B-type post-Asymptotic Giant Branch (AGB) stars to better understand their evolutionary status. Current theory proposes that the stars have undergone a gas-dust separation based on abundances obtained. Here, ROA 5701, in w Cen and Barnard 29, in M13, are investigated with high-resolution optical spectra. C, N, 0, Mg, AI, Si and S abundance estimates are determined, finding a general metal underabundance relative to young Galactic B-type stars. Their abundance patterns suggested that the stars have not undergone a gas-dust separation, although they may have evolved from the AGB before the third dredge-up. Gas-dust separation was suggested du~ to low Fe abundances, however, Fe abundances for these stars have previously only been estimated from ultraviolet (UV) spectra. Therefore, the role of optical Fe III absorption lines as abundance diagnostics is investigated, using optical spectra for Galactic B-type supergiants and mainsequence stars. With the observed Fe III spectra for a sample of the supergiants, and synthetic spectra from the model atmosphere codes TLUSTY and CMFGEN, non-LTE effects are found to be small. For a sample of the supergiants and main-sequence stars, LTE abundance estimates are obtained, consistent with previous optical studies and the Galactic environment. This thesis recommends a list of Fe III transitions for estimating reliable iron abundances from. early B-type stellar spectra. Following on from the optical Fe III study, for Barnard 29 and ROA 5701, iron abundances are found from both UV and optical spectra, with the UV abundances lower than those expected from the metallicities of the respective clusters. A similar systematic underabundance for UV Fe abundances isfound for other B-type stars in known metallicity environments, e.g. the Magellanic Clouds. The results indicate that Fe III UV lines may yield abundance values which are systematically too low by typically 0.6 dex and hence should be treated with caution.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.492341  DOI: Not available
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