Photometric identification of pulsation modes of B and A stars and a search for rotational effects on mode frequencies

Abstract: This thesis presents the results of an extensive observational research programme into the internal rotation dynamics of rapidly rotating stars. Several hundred hours of high quality multi-colour photometry and spectra were collected of 8 known pulsating stars suspected of being members of the β Cephei class. The recorded multi-colour photometry was combined with single-colour photometry from the ASAS, WASP and KELT projects and the pulsation spectra identified using Lomb-Scargle periodograms. The observing campaign was followed by intensive asteroseismological analysis using published grids of stellar models. For six of the eight observed stars, a stellar model was found that matched well in terms of the excited pulsations. In the process of the investigation, several novel techniques were developed in order to assist with the determination of the pulsation spectra and the search for well-matched stellar models. Several software libraries were also developed in order to assist with the data reduction and analysis process. These may be of interest to future asteroseismological studies as well as photometric studies in general. The investigation yielded 18 pulsation frequencies among the sample of 8 stars. Most of these had been observed during previous studies, again emphasising the need for space-based observing facilities in support of the study of pulsating stars. Of the 18 pulsation frequencies, 2 were identified with pulsation modes of high radial order (n > 5). Of particular interest in this regard is CPD-31◦ 6271, which exhibits two pulsation modes with n = 5 and one with n = 6. Fourteen of the observed pulsation frequencies were identified with pulsation modes of low degree (l ≤ 2), as is typical for β Cephei pulsators. No mode identifications could be made for the pulsations exhibited by HD 81589 and HD 37115. While the spectral type determinations of HD 81589 places it closest to the realm of δ Scuti pulsators, its pulsation spectrum is decidedly atypical for this class. Various aspects regarding the pulsational classification of this star are discussed. In the case of HD 37115, its rapid rotation prevented attempts to identify the observed pulsation frequencies with pulsation modes from non-rotating stellar model grids. Besides the non-identification of the pulsation modes in two of the stars, the chosen sample of pulsators agrees well with the population statistics of known β Cephei pulsators. The projected surface rotation speeds are consistent with β Cephei pulsators. In particular, the sample adheres to the observed correlation of decreasing maximum pulsation amplitude (in the Johnson V -filter) with increasing projected surface rotation speed. Among the 14 identified pulsation modes in 6 stars, no evidence of rotational splitting of pulsation eigenfrequencies could be detected. Given the excellent quality of the collected photometry, it is unlikely that the observed stars exhibit rotationally split pulsation multiplets with amplitudes that make observing them with ground-based observing facilities feasible. In fact, several pulsation frequencies were identified with amplitudes well below those of the rotationally split multiplets identified during other studies. This null result suggests that rotationally split pulsation frequencies of sufficient amplitude are rare among β Cephei pulsators. Studies of this kind are likely only feasible for a small number of stars in the absence of large photometric data sets from space-based observing facilities. Nevertheless, the results reported here will undoubtedly be of significant interest to the asteroseismological community.Ph.D. (Physics