Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Dr. John D. Landstreet

Abstract

The chemically peculiar magnetic A- and B-type (Ap/Bp) stars are characterised by large overabundances, of the order of 10^2 and 10^4 times the Sun, of Fe-peak and rare earth elements, respectively. Further, they possess strong, ordered magnetic fields (of order 1 kG) that are roughly dipolar in nature. We present in-depth investigations of magnetic Ap/Bp stars, ranging from detailed analyses of specific stars to larger surveys aimed at studying stratification and atmospheric abundance evolution.

For HD 133880 and HD 147010, we offer complete investigations of their magnetic fields and chemical abundance distributions. For each star, a simple magnetic field model is derived, from both line-of-sight and surface magnetic field measurements, that consists of dipole, quadrupole and octupole components. Abundance analysis is then performed using ZEEMAN, a spectrum synthesis program that takes into account the effects of the magnetic field.

Discrepant results between abundances derived from Si II and Si III in B-type stars has been documented in the literature. We report on the first comprehensive study of this phenomenon in various classes of B-type stars (including the non-magnetic HgMn and normal stars and magnetic Bp stars) ranging in effective temperature from about 11000 to 15000 K. We interpret the results in the context of vertical stratification in the atmospheres of these stars.

The abundance anomalies that exist in magnetic Ap/Bp stars are known to be produced by diffusion processes, when the gravitational settling of ions competes with radiative levitation. However, nothing is known about how these abundance anomalies may evolve during the main sequence lifetime of these stars. We present an extensive study of the atmospheric abundances for several elements of magnetic Ap/Bp stars, that are members of open clusters or associations (and therefore have well-determined ages), and attempt to interpret the results in the context of diffusion.


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