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The oscillation modes of Delta Scuti stars Kennelly, Edward James


Many S Scuti stars exhibit multiperiodic oscillations and are therefore considered to be good candidates for studies in stellar seismology; the modes of oscillations act as probes of the internal stellar structure. For S Scuti seismology to be successful, both the oscifiation modes and frequencies must be determined observationaily, then compared with theoreti cal models. Standard photometric observations can provide direct information only about the frequencies of those low-degree oscillations which give rise to large disk-integrated variations. In this thesis, oscillations of both low- and high-degree were studied by ana lyzing the variations introduced in the Doppler-broadened profiles of rapidly rotating S Scuti stars. The incidence of high-degree variability within the S Scuti instability strip was inves tigated with an spectroscopic survey of ~ 50 bright stars, carried out at the Dominion Astrophysical Observatory. Evidence for high-degree variations was discovered in 14 stars which were located both on the main sequence and in more evolved stages of evolution. A few stars (e.g., [formulas]) were investigated further with high-resolution (2.4 A/mm) spectra obtained at the Canada-France-Hawaii telescope. Using a twodimensional Fourier technique to transform the variations within the line profiles in both time and Doppler space, a representation of the variations was produced from which the apparent frequency and the apparent azimuthal order (or degree) could be directly determined. The technique was especially successful at uncovering multiple modes of oscillation. The observed mode spectra were found to be consistent with prograde, sec toral modes which oscillate with nearly equal frequency in the corotating frame of the star. However, the frequencies of the high-degree modes were lower than expected if they result from pressure waves trapped near the stellar surface. To study the oscillations in detail, observations lasting several days are required in order to provide adequate frequency resolution. As part of the 1992 MUSICOS campaign, 4 days of nearly continuous observations of [symbol] Tau were obtained from sites in China, France, the Canary Islands, and Kitt Peak. Analysis of the MUSICOS data has revealed several low- and high-degree modes (l≤ 8) at frequencies between 11 and 17 cycles day⁻¹.

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