On the Hα emission from the β Cephei system

Context.Be stars, which are characterised by intermittent emission in their hydrogen lines, are known to be fast rotators. This fast rotation is a requirement for the formation of a Keplerian disk, which in turn gives rise to the emission. However, the pulsating, magnetic B1IV star β Cephei is a very slow rotator that still shows Hα emission episodes like in other Be stars, contradicting current theories. Aims.We investigate the hypothesis that the Hα emission stems from the spectroscopically unresolved companion of β Cep. Methods.Spectra of the two unresolved components have been separated in the 6350–6850 Å range with spectro-astrometric techniques, using 11 longslit spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma. Results.We find that the Hα emission is not related to the primary in β Cep, but is due to its 3.4 mag fainter companion. This companion has been resolved by speckle techniques, but it remains unresolved by traditional spectroscopy. The emission extends from about -400 to +400 km s-1. The companion star in its 90-year orbit is likely to be a classical Be star with a spectral type around B6-8. Conclusions.By identifying its Be-star companion as the origin of the Hα emission behaviour, the enigma behind the Be status of the slow rotator β Cep has been resolved.