EDP Sciences
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Volume 497, Number 2, April II 2009
Page(s) 487 - 495
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361/200811105
Published online 05 March 2009
A&A 497, 487-495 (2009)
DOI: 10.1051/0004-6361/200811105

The close Be star companion of $\mathrm \beta$ Cephei

H. E. Wheelwright1, R. D. Oudmaijer1, and R. S. Schnerr2

1  The School of Physics and Astronomy, EC Stoner Building, The University of Leeds, Leeds LS2 9JT, UK
    e-mail: pyhew@leeds.ac.uk
2  Institute for Solar Physics, Royal Swedish Academy of Sciences, Albanova University Centre, 10691 Stockholm, Sweden

Received 7 October 2008 / Accepted 16 February 2009

Context. The prototype of the $\mathrm \beta$ Cephei class of pulsating stars, $\mathrm \beta$ Cep, rotates relatively slowly, and yet displays episodic H$\alpha$ emission. Such behaviour is typical of a rapidly rotating, classical Be star. For some time this posed a contradiction to our understanding of the Be phenomena as rapid rotation is thought to be a prerequisite for the characteristic emission phases of Be stars. Recent work has demonstrated that the H$\alpha$ emission is in fact due to a close companion (separation $\approx$ $ 0.25\arcsec$) of the star. This resolves the apparent enigma if this close companion is indeed a classical Be star, as has been proposed.
Aims. We aim to test the hypothesis that this close companion is a valid Be star by determining properties such as its spectral type and $v \sin i$.
Methods. We employed the technique of spectroastrometry to investigate the close binary system. Using the spectroastrometric signatures observed, we split the composite binary spectra into its constituent spectra in the B band (4200–5000 Å) and R band (6200–7000 Å).
Results. The spectroastrometrically split spectra allow us to estimate spectral types of the binary components. We find that the primary of the close binary system has a spectral type of B2III and the secondary a spectral type of B5Ve. From the relationship between mass and spectral type, we determine the masses of the binary components to be $M_{\rm pri} = 12.6 \pm 3.2~M_{\sun}$ and $M_{\rm sec} = 4.4 \pm 0.7~M_{\sun}$ respectively. The spectroastrometric data allow some constraint on the orbit, and we suggest a moderate revision to the previously determined orbit. We confirm that the primary of the system is a slow rotator ( $v \sin i = 29^{+43}_{-29}$ km s-1), while the secondary rotates significantly faster, at a $v \sin i = 230 \pm 45$ km s-1
Conclusions. We show that the close companion to the $\mathrm \beta$ Cephei primary is certainly a valid classical Be star. It has a spectral type of B5Ve and is a relatively fast rotator. We confirm that the $\beta$ Cephei system does not contradict our current understanding of classical Be stars.

Key words: binaries: close -- binaries: general -- stars: emission-line, Be -- stars: individual: $\mathrm \beta$ Cephei

© ESO 2009