Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The rapidly oscillating Ap (roAp) star HD 60435 has been monitored in a programme of rapid Johnson B photometry during which approximately 355 hours of observations were collected from the Las Campanas, Cerro Tololo Inter-American and South African Astronomical Observatories in two coordinated campaigns from January 1984 to March 1985.;Fourier analysis of these data indicate that the star undergoes oscillations in a spectrum of nearly equally spaced frequencies, corresponding to a range of periods from about 12 to 20 minutes. Typical amplitudes are only a few millimagnitudes. The dominant oscillations occur near a frequency of 1.4 mHz (P {dollar}\simeq{dollar} 12 min). Fourier peaks at frequencies near 2.8 and 3.6 mHz have also been detected sporadically; these may be 2:1 and 3:1 resonances with the 1.4 mHz oscillation, or harmonics produced by the frequency analysis.;Comparison of the overall frequency pattern to the asymptotic theory of nonradial acoustic pulsation has led to the identification of the oscillations as a series of high-overtone (13 {dollar}\le n \le{dollar} 28) p -modes of degree {dollar}\ell{dollar} = 1 and 2. The fundamental frequency spacing {dollar}\nu\sb{lcub}\rm o{rcub} \simeq{dollar} 52 {dollar}\mu{dollar}Hz is consistent with a slightly evolved late A star whose radius is 2.2 {dollar}\pm{dollar} 0.3 R{dollar}\sb\odot{dollar}, as are the results of the classification spectra and Stromgren multi-colour photometry of the star.;The amplitudes of the dominant oscillations are modulated over a long timescale near 8 days, and also show rapid modulation in only a few hours. The latter is primarily due to beating among the many frequencies present in HD 60435, but there is also evidence of actual growth and decay of pulsation modes in less than a day.;The rotation period of HD 60435, based on the star's long-term light curve, is 7.6662 {dollar}\pm{dollar} 0.0001 days. This agrees with the modulation period of the oscillations, as is predicted by the oblique pulsator model for the roAp variables. Application of a dynamical version of this model to the fine splitting observed in the Fourier spectrum of the oscillations places constraints on the magnetic field geometry, predicts that the field should exhibit polarity reversal, and suggests that HD 60435 may have a relatively weak internal field compared to other roAp stars.



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