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Astron. Astrophys. 327, 640-647

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Spectroscopic observations of the interacting binary BY Crucis*

K. Daems** and C. Waelkens

Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium

Received 15 April 1997 / Accepted 17 June 1997


We present multi-wavelength spectroscopic data of the 106 day period interacting binary BY Crucis (Daems et al. 1997). The system consist of an early F-type supergiant that fills its Roche lobe and a more massive companion, hidden in a thick accretion disc.

In the far-UV spectra (from 1200 Åto 1950 Å), strong emission lines of highly ionised elements are observed, superposed on a continuum. Both components of the UV flux originate from the accretion disc around the companion. In the UV range from 1950 Åto 3150 Åthe radiation of the F-type mass donor is dominant.

In the visual and the red range we focused our interest on the Balmer lines (HFORMULA, HFORMULA and HFORMULA), He I 5876 Å, Na I 5890-5896 Å, and the forbidden line [O I] 6300 Å. We also obtained spectra in various other regions, ranging from 3900 Å to 6700 Å. Evidence of mass transfer is observed in the He I-line, the Na I-lines, HFORMULA and some Fe-lines. The HFORMULA-line shows no variability, and its double peaked profile implies that it is formed in a circumbinary disc or envelope. The [O I] line confirms the presence of matter around the binary system. By analysing the line profile variability of He I, we construct a tentative image of the gas flow in the accretion disc around the unseen companion. The Balmer lines are used to probe the outer, cooler gas, flowing through the outer Lagrangian point into a circumstellar disc or envelope.

Key words: binaries: close - circumstellar matter - stars: evolution - stars: individual: BY Cru

*Based on observations obtained with the IUE satellite; and at the European Southern Observatory, La Silla, Chile.
**Research Assistant of the Fund for Scientific Research - Flanders

Send offprint requests to: K. Daems, Leuven

© European Southern Observatory (ESO) 1997

Online publication: October 14, 1997
Last change: April 6, 1998