The pulsation of the outer layers of the BET CEP star SIG Sco.

Abstract

The pulsation of the outer layers of the Beta Cephei-type variable σ Sco is investigated using 17 ultraviolet spectrograms (1200-2000 A) obtained during seven hours of observations with the International Ultraviolet Explorer. A pulsational model established earlier for BW Vul is confirmed. According to this model, after a period of photospheric rest the outer layers expand suddenly. The acceleration for the outermost layers, investigated with the resonance lines of C IV and Si IV, is found to be larger than for the photosphere. During the phase of deceleration that follows, no falling down of the C IV absorbing layers is observed; their velocity component remains directed outward. A transient stellar wind component sets in just before the star has its minimum radius (phase, approximately 0.9); it is strongest at a phase of approximately 0.2. The observations are interpreted as suggesting the mechanism of pulsational driven mass loss.