Smith, Keith Colin; (1992) The chemical compositions of mercury-manganese stars from ultraviolet spectra. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is primarily an investigation of the chemical compositions of normal and mercury-manganese (HgMn) type late-B stars by analysis of their ultraviolet and visual region spectra. Refined techniques of co-addition and cross correlation are employed in the preparation of a dataset of relatively high signal-to-noise, high resolution spectra from archival International Ultraviolet Explorer (IUE) data for forty programme stars. Fourier analysis is used in a study of the photometric noise properties and instrumental response function of these IUE spectra, and in new determinations of the rotational velocities of selected programme stars. The effective temperatures and surface gravities of the programme stars are derived from published Strömgren and Geneva photometric data, and by fitting the predictions of fully line-blanketed model atmospheres to observed stellar energy distributions and hydrogen Balmer line profiles. Calculations are carried out in order to explore the effects of interstellar reddening and spectrographic scattered light on the diagnostic properties of these observations. The validity of the use of normal helium abundance, solar metallicity model atmospheres in the analysis of HgMn stars is confirmed. Rotational velocities, microturbulence parameters, and elemental abundances are derived for a small selection of the programme stars by analysis of high dispersion photographic spectra. Evidence is presented for the existence of a sub-class of mild HgMn stars. The abundances of seventeen elements in the programme stars are determined by detailed spectrum synthesis analysis of their co-added ultraviolet spectra. A bivariate statistical analysis of these abundances is carried out and significant correlations and inter-correlations between atmospheric parameters and elements are identified. Profound violations of nucleosynthetic systematics are found in the abundances of the HgMn stars at manganese, copper, and gallium. The predictions of the parameter-free radiative diffusion model for HgMn stars are shown to be in good agreement with the observed abundances, particularly in the case of manganese for which the effects of atmospheric stratification are considered in detail.

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