The close Be star companion of Beta Cephei

Abstract

The prototype of the Beta Cep class of pulsating stars, Beta Cep, rotates relatively slowly, and yet displays episodic Halpha emission. Such behaviour is typical of a 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 emission phases of Be stars. Recent work has demonstrated that the Halpha emission is in fact due to a close companion (separation ~0.25") of the star. This resolves the apparent enigma if this close companion is a classical Be star. We aim to test the hypothesis that this close companion is a valid Be star by determining properties such as its spectral type and vsin i. We employed 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 and R band. The 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_p = 12.6+/-3.2 M_* and M_s = 4.4+/-0.7 M_* 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 (vsin i =29 +43/-29 km/s), while the secondary rotates significantly faster, at a vsin i =230+/-45 km/s. We show that the close companion to the beta Cep primary is a valid classical Be star. It has a spectral type of B5Ve and is a relatively fast rotator.

arXiv commentAccepted for publication in A & A, 9 pages, 8 figures

References in corpus (9)

• Be-star rotation: how close to critical?
• Catalog of Galactic Beta Cephei Stars
• On the binarity of Herbig Ae/Be stars
• A Statistical Study of Threshold Rotation Rates for the Formation of Disks around Be Stars
• Abundance analysis of prime B-type targets for asteroseismology I. Nitrogen excess in slowly-rotating beta Cephei stars
• On the spectroastrometric separation of binary point-source fluxes
• On the H$\alpha$ emission from the $\beta$ Cephei system
• Sub-milliarcsecond precision spectro-astrometry of Be stars
• H$\alpha$ long term monitoring of the Be star $\beta$ Cep Aa

Cited by in corpus (5)

• The mass ratio and formation mechanisms of Herbig Ae/Be star binary systems
• Discovery of the magnetic field in the pulsating B star beta Cephei
• A JVLA survey of the high frequency radio emission of the massive magnetic B- and O-type stars
• Surveying the Bright Stars by Optical Interferometry III: A Magnitude-Limited Multiplicity Survey of Classical Be-Stars
• An investigation of the close environment of beta Cep with the VEGA/CHARA interferometer