P Cygni as an unstably oscillating hypergiant

A.M. van Genderen, C. Sterken & M. de Groot

Nearly 20 years of photometric observations of P Cygni allowed the identification of a number of oscillations amongst others a very weak S Dor (SD)-type variability and two types of micropulsations typical for S Dor variables (or LBVs) (de Groot et al. 2001b). These results are discussed in the framework of several studies of de Jager and collaborators on the diagnosis of the types of stellar oscillations, on the physics of the atmosphere of P Cyg and on the two instability regions on the upper HR-diagram, one for the yellow hypergiants and one for the S Dor variables. It is speculated that the "100d-type" optical microvariations, the recurrent shells and the long-period `discrete absorption components' (DACs) in the spectral lines are related. It seems that the depth of the sonic point, as computed by de Jager et al. (2001) is one of the necessary conditions to cause strong instabilities in S Dor variables resulting in large light amplitudes during SD-phases.

Manuscript: jad8_8g.ps