The close Be star companion of Cephei
The School of Physics and Astronomy, EC Stoner Building, The University of Leeds, Leeds LS2 9JT, UK e-mail: firstname.lastname@example.org
2 Institute for Solar Physics, Royal Swedish Academy of Sciences, Albanova University Centre, 10691 Stockholm, Sweden
Accepted: 16 February 2009
Context. The prototype of the Cephei class of pulsating stars, Cep, rotates relatively slowly, and yet displays episodic Hα 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 Hα emission is in fact due to a close companion (separation ≈) 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 .
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 and 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 ( km s-1), while the secondary rotates significantly faster, at a km s-1
Conclusions. We show that the close companion to the 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 β Cephei system does not contradict our current understanding of classical Be stars.
Key words: binaries: close / binaries: general / stars: emission-line, Be / stars: individual: β Cephei
© ESO, 2009