Long-term spectroscopic monitoring of BA-type supergiants. I. H_alpha_ line-profile variability.

Abstract

We have obtained time series of spectra in the wavelength range 4000-6800 A over several months with high S/N and high resolution in wavelength (λ/{DELTA}λ=~20000) and time ({DELTA}t=~1d) of the late-type B and early-type A supergiants HD91619 (B7Ia), βOri (B8Ia), HD96919 (B9Ia), HD92207 (A0Ia), HD100262 (A2Ia) and αCyg (A2Ia). H_alpha_ is found to show broad emission extended to about +/-1200km/s for all objects except αCyg. Due to the lack of strong line-emission in H_alpha_ the electron-scattered photons are expected to originate in deep atmospheric layers. In all of the objects the H_alpha_-line profiles are found to be highly variable on different time scales reaching from days to months. Patterns of variation in H_alpha_ are found to be quite symmetric about the systemic velocity and are mainly due to variable blue and red-shifted emission superimposed on almost constant photospheric and/or wind profiles. These V/R variations are interpreted in terms of axial symmetry of the envelopes of these objects. Time-series analyses of the variations reveal H_alpha_ time scales up to a factor of 6 longer than expected radial fundamental pulsation periods but consistent with rotational periods. Therefore, rotational modulation as a possible source of variability is concluded. Corotating weak magnetic surface structures are suggested as the source for a rotationally modulated lower wind region. Suddenly appearing deep and highly blue-shifted absorptions in H_alpha_ are ascribed to instabilities of the ionization structure of the wind. Outwards propagating discrete absorption components have been observed only once in HD92207.