Gravity modes and potential evidence for Rossby Waves in late O-type supergiants
The properties of O-type supergiant stars remain largely unexplored. By analysing their light variations, we can unravel underlying physical phenomena, such as binarity, stellar pulsations, and rotation modes. This study aims to analyse the TESS light curves of 3 O-type supergiants to identify periodic signatures and gain deeper insights into their internal dynamics and structure. Our primary goal is to search for the presence of rotational modulation and possible evidence of Rossby waves. A period search was performed and we explored phase diagrams and searched for rotational splitting. If the observed wave frequencies were consistent with the dispersion relation of global Rossby waves, we identified them as potential signatures of this mechanism. We used a graphical method to compare the observed and predicted frequencies. The analysed stars (HD 188001, HD 192639, and HD 159952) exhibit a comparable set of frequencies. We classify most of them as g-mode oscillations (either l=1 or l=2), in agreement with the evolutionary state of the objects. In all cases, we find evidence of rotational modulation. The remaining oscillation patterns may be consistent with Rossby modes, including both tesseral and sectoral configurations. The angle of inclination of the rotation axis was estimated using stellar parameters available in the literature, together with the rotational period derived in this work. We also discuss a possible connection between the observed low-frequency waves and the red-noise component commonly observed in the periodogram of photometric time series of massive supergiants. Our results provide evidence of g-mode oscillations modulated by rotation. In addition, we find that Rossby waves may be excited in rotating O-type supergiants, which suggests that these large-scale inertial oscillations could play a role in the observed low-frequency variability of such stars.