EDP Sciences
Free access
Volume 474, Number 1, October IV 2007
Page(s) 229 - 237
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20077308

A&A 474, 229-237 (2007)
DOI: 10.1051/0004-6361:20077308

Multi-component absorption lines in the HST spectra of $\alpha$ Scorpii B

R. Baade and D. Reimers

Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
    e-mail: rbaade@hs.uni-hamburg.de

(Received 15 February 2007 / Accepted 9 July 2007)

Context.Extended atmospheres of late-type supergiants are of special interest with regard to the mass-loss process, the circumstellar physico-chemistry, and the evolution of stars.
Aims.The circumstellar envelope of $\alpha$ Sco A (M1.5 Iab) has been subject of intensive studies. Nevertheless the mechanism that is responsible for the mass outflow is still poorly understood. High-resolution spectroscopy is required to re-investigate the wind properties of the supergiant.
Methods.We present HST/GHRS observation of $\alpha$ Sco B (B2.5 V) showing numerous absorption lines that arises from matter being ejected by the supergiant primary. Multiple absorption features indicates that the wind material is not smoothly distributed but rather concentrated in discrete condensations. The column density and velocity structure of the absorption components is estimated using a nonlinear least-squares fit procedure.
Results.We have identified four distinct absorption systems with rest frame velocities of -0.5, -8.4, -14.4, and -19.9 km s-1. An additional component is seen in Al III at 7.8 km s-1 probably constraining the position of the hot companion. With a spherical outflow geometry we would obtain a time average mass-loss rate due to episodic events up to $\dot{M}$ = 10-5 $M_{\odot}$ yr-1. The continuous background flow leads to a significant smaller mass-loss rate of 3 $\times$ 10-7 $M_{\odot}$ yr-1.
Conclusions.There are two alternative explanations for the observed features: Either a moderate continuous mass flow is accompanied by episodic mass-loss events forming concentric shells; or the continuous outflow is disturbed by discrete clumps due to instabilities or unknown ejection processes. As a consequence former mass-loss determinations should be interpreted with care.

Key words: binaries: visual -- circumstellar matter -- stars: mass-loss -- stars: late-type -- stars: individual: $\alpha$ Scorpii

© ESO 2007