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Astron. Astrophys. 318, 157-170 (1997)

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Wind variability of B supergiants

II. The two-component stellar wind of [FORMULA] Arae

R.K. Prinja 1, D. Massa 2, A.W. Fullerton 3, I.D. Howarth 1 and M. Pontefract 1

1 Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
2 Applied Research Corporation, 8201 Corporate Drive, Landover, MD 20786, USA
3 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, Postfach 15 23, D-85740 Garching bei München, Germany

Received 14 March 1996 / Accepted 17 June 1996

Abstract

The stellar wind of the rapidly rotating early-B supergiant, [FORMULA] Ara, is studied using time series, high-resolution IUE spectroscopy secured over [FORMULA] days in 1993 March. Results are presented based on an analysis of several line species, including N V, C IV, Si IV, Si III, C II, and Al III. Comparisons of the time-averaged wind line morphology of [FORMULA] Ara to the UV spectra of other OB stars, and to profiles from spherically symmetric wind models, suggest that the wind of [FORMULA] Ara is equatorially enhanced.

Co-existing time variable features are identified at low-velocity (redward of [FORMULA] km s-1) and at higher-speeds extending to [FORMULA] km s-1. The observed interface between these structures is 'defined' by the appearance of a discrete absorption component which is extremely sharp (in velocity space). The central velocity of this 'Super DAC' changes only gradually, over several days, between [FORMULA] and -750 km s-1 in most of the ions. However, its location is shifted redward by almost 400 km s-1 in Al III and C II, indicating that the physical structure giving rise to this feature has a substantial velocity and ionization jump. Constraints on the relative ionization properties of the wind structures are discussed. The overall wind activity in [FORMULA] Ara exhibits a clear ion dependence, such that low-speed features are promoted in low-ionization species, including Al III, C II, and Si III. We also highlight that - in contrast to most OB stars - there are substantial differences in the epoch-to-epoch time-averaged wind profiles of [FORMULA] Ara and that the high-speed component observed during our 1993 time series is normally not present.

We discuss the potential roles of the radiative bi-stability mechanism and wind compressed regions due to rapid stellar rotation for providing gross wind distortions in [FORMULA] Ara.

Key words: line: profiles – stars: early-type – stars: individual (HD 157246) – stars: mass loss

Send offprint requests to: R.K. Prinja

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998
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