Variability of Be stars

This study explores the interplay between non-radial pulsations (NRPs) and the formation and variability of circumstellar discs in Be stars through a combined photometric and spectroscopic analysis. NRPs are hypothesized to play a critical role in the ejection of stellar mass during outbursts, significantly influencing disc formation and variability, particularly in emission lines such as Hα and in photometric behaviour. By integrating photometric data from the TESS space telescope with spectroscopic data from the BeSS database, this thesis investigates the hypothesized correlations between NRPs and circumstellar disc dynamics. While no clear correlation was found between NRP influencing the creation and dissipation of the disc (as probed by Hα emission), on the other hand, we found that stars with higher NRP amplitude do not necessarily relate to host stronger discs. Highlighting the complexity of Be star systems and the need for multi-wavelength studies to unravel their variability. The analysis encompassed 134 Be stars, with over 220 TESS light curves and 560 Hα spectra collected. Spectroscopic data were synchronized with TESS observations to enable simultaneous analysis of light curves and spectral variability. Stellar parameters, including effective temperature (Te f f ), surface gravity (log g), and rotational velocity (v sin i), among others, were catalogued. Light curves were extracted and processed using the Python package lightkurve. Frequency analysis was conducted using the Lomb-Scargle method, producing periodograms for each star to identify characteristic frequencies. A focused study on six Be stars (f01 Cyg, HD 45314, HD 110432, HD 120991, HD 174237, and ζ Tauri) was performed due to their pronounced circumstellar disc features, variability in light curves and spectra, and their classification as γ Cassiopeiae analogues. These stars, primarily early-type Be stars (B0-B2), exhibited g-mode pulsations with frequencies ranging between 0.8 and 4 d−1, alongside complex pulsational behaviour. Notably, HD 110432 demonstrated a unique single p-mode pulsation, while HD 45314, an Oe star, displayed distinctive characteristics. Orbital periods were recovered for the two binary systems f01 Cyg and HD 174237. For f01 Cyg, the disc’s radial extent was estimated at ∼ 15, R⊙, well within the Roche limit (∼ 40, R⊙). HD 120991 exhibited high-intensity emission, which was attributed to an outburst, with periodic V/R ratio variations detected during emission peaks. In HD 110432, amplitude variations in pulsational frequencies were interpreted as early signs of a future outburst. However, no definitive correlation was found between NRPs and Hα line variability (formation or dissipation of the Be star disc), highlighting the need for high- resolution, high-cadence spectroscopic observations, and also the highly complex nature of these kinds of stars. The results contribute to a broader understanding of Be star variability and disc dynamics. It highlights the potential of combining photometric and spectroscopic datasets to unravel short-term variability patterns, thereby advancing the study of disc formation and dissipation processes.