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Astron. Astrophys. 326, 237-248 (1997)

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Emission features in Br [FORMULA] and Br [FORMULA] spectra of normal O and B stars *

P.A. Zaal 1, L.B.F.M. Waters 1, 2, T.R. Geballe 3 and J.M. Marlborough 4

1 Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 SRON Laboratory for Space Research Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
3 Joint Astronomy Center, 660 N A'ohoku Place, Hilo, Hawaii 96720, USA
4 Department of Physics and Astronomy, University of Western Ontario, London, Ontario, N6A 3K7, Canada

Received 6 November 1996 / Accepted 15 April 1997


We present high resolution Br [FORMULA] and Br [FORMULA] spectra for 15 near main-sequence O and B stars in a broad range in [FORMULA] and with spectral type between O9-B2. The HI infrared lines probe the outer regions in the atmosphere and the onset of the stellar wind. The slowly rotating stars (with [FORMULA] [FORMULA] 50 kms-1) show weak, single-peaked emission features on top of a broad absorption line. The most likely explanation is a non-LTE effect in the outer photosphere causing emission at line center. The slow rotators include two [FORMULA] Cephei stars, both of which also show HeI (4.049 µm) emission. This indicates a significant increase in the degree of ionization in the outer layers of these B2-3 stars. For the higher [FORMULA] stars we find two cases (out of six) in which weak, double-peaked emission is visible on top of the photospheric absorption. These two stars probably have low-density discs which are apparent only in the infrared HI lines (Zaal et al., 1995).

Key words: stars: atmospheres – stars: early-type – stars: circumstellar matter – infrared: stars – line: formation

* Data published here were obtained at the UK Infrared Telescope (UKIRT) which is operated by the Royal Observatory, Edinburgh on behalf of the Science and Engineering Research Council.

Send offprint requests to: P.A. Zaal

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998