A photometric and spectroscopic study of the variability of the O7.5 supergiant HD192639

Abstract

This thesis investigates the photometric and spectroscopic variability of the O7.5 Iabf-type supergiant star HD 192639, utilizing data from TESS and the TIGRE telescope. Previous studies identified a potential 4.76-day period in the stellar wind variability, potentially linked to large-scale structures or magnetic activity. This work aims to confirm or refute this period using new observational datasets, including simultaneous photometric and spectroscopic data, a first for this object. Fourier analysis and Temporal Variance Spectra (TVS) reveal no statistically significant persistent periodicity in the TESS light curves, though a marginal signal consistent with the 4.76-day timescale appears repeatedly in both photometry and spectroscopy. Spectroscopic analysis confirms significant variability in several wind-sensitive lines (notably He II 4686, Hβ, He I 5876, and Hα) consistent with large-scale wind structures. However, TVS for photospheric lines such as He II 4200 only show stochastic variations. Radial velocity measurements, performed on photospheric spectral lines, yield no evidence for binarity. Hence, the observed variability is most likely intrinsic to the O7.5 supergiant and does not arise from binary interactions. Although the variations of HD 192639 are clearly not strictly periodic, our results support the idea that the 4.76-day timescale reflects a stable clock, most probably associated with the stellar rotation. Localized magnetic fields, leading to bright spots, are likely at the origin of the large-scale wind structures. A very special behaviour was observed during the night of July 27–28, 2021: wind-sensitive spectral lines displayed a blueward shifting narrow absorption component. This coincides with an anomalously strong photometric dip, suggesting an episodic enhanced mass ejection event. Overall, these findings refine our understanding of HD 192639’s wind structure and variability, and underline the added value of long-term, high-cadence monitoring to unravel the dynamics of massive star winds.