Variability and the nature of luminous stars. The case of Schulte 12

The nature of luminous evolved massive stars is still under debate. Observationally determined constraints are important arguments in this discussion. We have investigated the photometric and spectroscopic variability pattern of the blue hypergiant CygOB2 #12 (Schulte12) in the timescale from a few days to several months. We have found that 1) the violet-to-red variability of the Hα emission profile over 3...4 months points to the essential asymmetry of stars gaseous envelope or stellar wind, 2) the correlated day to day variability of stellar brightness and intensity of absorption lines (HeI, CII) demonstrates the irregular patchy structure of the visible stellar disk caused by pulsations or large scale turbulent cells. These evidences stress that the common assumptions on the spherically symmetric wind and on the stability of photospheric physical parameters (e.g. T_{eff}) may not be justified, and that the variability of boundary conditions should be an obligatory ingredient of the modelling of outer layers in CygOB2 #12 and obviously, in other similar stars.