An Abundance Survey of the Ap stars of the Southern Hemisphere

Hall, Martin (2020) An Abundance Survey of the Ap stars of the Southern Hemisphere. Doctoral thesis, University of Central Lancashire.

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The atmospheres of Chemically Peculiar A stars provide a fascinating area of re- search on many levels: their slow rotation, strong magnetic fields and lack of turbulent convection give rise to stratification of different ionic species of the same element through a process of radiative levitation. In particular, this phenomenon has already been seen in the abundance disequilibria of Iron, and rare earth elements such as Neodymium (Nd) and Praseodymium (Pr). The research work described in this thesis involves the development of a semi-automated set of routines coded in the programming language IDL to measure equivalent widths of selected Fe lines, and the application of these routines to perform an abundance survey of Fe i and Fe ii species on the high-resolution spectra of approximately 350 Ap stars collected between 2007 and 2010 on the FEROS Echelle spectrograph housed at the 2.2–m telescope at European Southern Observatory at La Silla, Chile. The results show strong correlations with effective temperature as a consequence of atmospheric strat- ification. The approach is then extended to the ionic pairs of rare earth species Pr ii / Priii and Ndii / Ndiii to investigate whether these abundance disequilibria in the rapidly oscillating Ap stars are a consequence of the relatively cool temperatures of those stars, or rather a signature of pulsation. We believe that the onset of this disequilibrium phenomenon observed in the cooler Ap stars is more likely to be a temperature effect, rather than a signature of pulsation.

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