The close Be star companion of β Cephei
Abstract
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- April 2009
- DOI:
- 10.1051/0004-6361/200811105
- arXiv:
- arXiv:0902.4356
- Bibcode:
- 2009A&A...497..487W
- Keywords:
-
- binaries: close;
- binaries: general;
- stars: emission-line;
- Be;
- stars: individual: β Cephei;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in A &