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

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

2006-10-06

arxiv.org/abs/astro-ph/0610198v1

Astronomy and Astrophysics, Volume 459, Issue 1, November III 2006, pp.L21-L24

Astronomy and Astrophysics

Astrophysics

4 pages, 3 figures. Accepted by A&A Letters

Scientific paper

10.1051/0004-6361:20066392

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

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