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The rapidly oscillating Ap star HR 1217 : the effect of a magnetic field on pulsation Cameron, Christopher J.

Abstract

The rapidly oscillating Ap (roAp) stars provide a unique opportunity to observe and test processes that until recently could only be studied in detail for the Sun. As a class, these stars possess extreme examples of chemical inhomogeneities through the effects of gravitational settling, mixing and radiative accelerations; magnetic fields that affect both the micro- and macroscopic physics that influence stellar structure; and high overtone pulsations. For the latter, we may use the tools developed for asteroseismic analysis: the inference of the internal structural properties of a star based on information from its pulsational instabilities. One of the most studied of the roAp stars is HR 1217. The rapid variability of this star was first discovered by Kurtz (1982). Later, Kurtz et al. (1989) extended photometric observations of HR 1217 during a global campaign. They discovered six primary frequencies with periods near 6 minutes and a spacing pattern that is reminiscent of pulsations observed in the Sun. This thesis presents a revised frequency analysis of photometric data on HR 1217 obtained during a Whole Earth Telescope (WET) campaign in late 2000. In particular, we identify a new frequency at 2788.94 μHz with a characteristic second-order p-mode spacing of 2.63 μHz from another frequency previously reported by Kurtz et al. (2002). Evolutionary and pulsational models of A stars are also presented with a discussion of how a magnetic field can perturb the acoustic frequencies. To accomplish this, the variational method used by Cunha & Gough (2000) is adapted to calculate magnetic perturbations to the acoustic modes calculated from our evolutionary models. These perturbations are used to explain the unexpected frequency spacing observed in HR 1217.

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