Long-term spectroscopic monitoring of BA-type supergiants. II. High-velocity absorptions in βOri and HD96919.

During our extended monitoring campaigns on late B and early A-type supergiants (Kaufer et al. 1996A&A...305..887K, Paper I) we have observed extraordinarily deep and highly blue-shifted absorption events in the Hα line. In this work, new time-series observations showing the most extreme cases of such events observed so far are presented for two objects, βOri (B8Ia) and HD96919 (B9Ia). The development of these high-velocity absorption (HVA) events in velocity and time are discussed: the HVAs show no signs of spherically symmetric mass-loss events with subsequent accelerated propagation into the wind. The absence of unshifted line emission in connection with the HVAs is especially indicative of the non-sphericity of the active circumstellar regions. Simultaneously with the blue-shifted absorption, red-shifted absorption is found in Balmer and metallic lines, primarily during the onset of the developing event, which clearly reveals the complex structure of the involved velocity fields. Mass outflow and mass infall are present in the envelope. As a picture for the circumstellar structures that cause the sudden appearance of the HVAs over a large velocity range, localized regions of enhanced mass loss on the stellar surface, which build up extended, rotating streak lines in the equatorial plane are suggested. Finally, the role of a critical ionization structure in the condensed structures is discussed.