The Blue Supergiant (BSG) phase is a transient, hence very short epoch of evolution of massive stars. Therefore, the study of the internal structure of these core collapse, SN II progenitors can provide unprecedented information about pre-collapse conditions near the stellar core, as well as providing insights on how massive stars evolve on the main sequence. Here we present 28 days of high precision MOST photometry and >6 years of radial velocity monitoring of the blue supergiant star Rigel (β Ori, V~0.09 mag; B8 Ia) which has similar properties to the prototype of the α Cyg variable class Deneb. Rigel is well suited for such a study since it is the nearest BSG and SN-II progenitor with well-defined physical properties. Nineteen significant pulsation modes (with SNR >= 4.6) are reported from our study. We detect variability time scales ranging from a few days up to months. These appear to be associated with very high order, low degree gravity modes.
Our rotating stellar structure and evolution model (MESA) shows that the star at its current location on the Hertzsprung-Russel Diagram is fusing Helium in its convective core in addition to CNO fusion in a radiative shell surrounding the core.
Linear fully non-adiabatic non-radial stability analysis (GraCo) results in the excitation of a limited number of non-radial (1 <= l <= 3) mixed gravity modes. The Fundamental radial mode (l = 0) and its overtones are all stable.
We find the conditions in the Hydrogen burning shell (which is located below the intermediate convective zone) favorable for destabilization of g-modes through ??mechanism, supported by the logarithmic temperature derivative of nuclear energy generation rate 11.0 ? ?T ? 14.0. The ??mechanism has been found to be a successful means of explaining oscillations found in compact white dwarfs, but we show that this mechanism can also be applied to evolved BSGs. Only pulsational instabilities with periods ranging between 26 to 120 days can be explained with this mechanism and the origin of the short-period variations in Rigel still remain not fully explained.
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