On the H$α$ emission from the $β$ Cephei system

Abstract

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 H$α$ emission episodes like in other Be stars, contradicting current theories. We investigate the hypothesis that the H$α$ emission stems from the spectroscopically unresolved companion of $β$ Cep. Spectra of the two unresolved components have been separated in the 6350-6850Årange with spectro-astrometric techniques, using 11 longslit spectra obtained with ALFOSC at the Nordic Optical Telescope, La Palma. We find that the H$α$ emission is not related to the primary in $β$ Cep, but is due to its 3.4 magnitudes 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}$. The companion star in its 90-year orbit is likely to be a classical Be star with a spectral type around B6-8. By identifying its Be-star companion as the origin of the H$α$ emission behaviour, the enigma behind the Be status of the slow rotator $β$ Cep has been resolved.