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The close Be star companion of β Cephei
Stockholm University, Faculty of Science, Department of Astronomy. (Institute for Solar Physics)
2009 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 497, 487-495 p.Article in journal (Refereed) Published
Abstract [en]

Context: The prototype of the β Cephei class of pulsating stars, β 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 ≈ 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_pri = 12.6 ± 3.2 Msun and M_sec = 4.4 ± 0.7 Msun 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 ± 45 km s-1 Conclusions: We show that the close companion to the β 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.

Place, publisher, year, edition, pages
2009. Vol. 497, 487-495 p.
Keyword [en]
binaries: close, binaries: general, stars: emission-line, Be, stars: individual: &beta, Cephei
URN: urn:nbn:se:su:diva-35883ISBN: 0004-6361OAI: diva2:288438
Available from: 2010-01-21 Created: 2010-01-20 Last updated: 2011-02-15Bibliographically approved

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