The close-in companion of the fast rotating Be star Achernar

Abstract

Context: Be stars are massive dwarf or subgiant stars that present temporary emission lines in their spectrum, and particularly in the Halpha line. The mechanism triggering these Be episodes is currently unknown, but binarity could play an important role. Aims: Previous observations with the VLT/VISIR instrument (Kervella & Domiciano de Souza 2007) revealed a faint companion to Achernar, the brightest Be star in the sky. The present observations are intended to characterize the physical nature of this object. Methods: We obtained near-IR images and an H-band spectrum of Achernar B using the VLT/NACO adaptive optics systems. Results: Our images clearly show the displacement of Achernar B over a portion of its orbit around Achernar A. Although these data are insuficient to derive the orbital parameters, they indicate a period of the order of 15 yr. The projected angular separation of the two objects in December 2007 was smaller than 0.15 arcsec, or 6.7 AU at the distance of Achernar. Conclusions: From its flux distribution in the near- and thermal-IR, Achernar B is most likely an A1V-A3V star. Its orbital period appears similar to the observed pseudo-periodicity of the Be phenomenon of Achernar. This indicates that an interaction between A and B at periastron could be the trigger of the Be episodes.

arXiv commentAccepted for publication as an A&A Letter. Astronomy and Astrophysics (2008) In press

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• Short-term variability and mass loss in Be stars IV. Two groups of closely spaced, approximately equidistant frequencies in three decades of space photometry of $\nu$ Puppis (B7-8 IIIe)
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• The binary system of the spinning-top Be star Achernar
• The environment of the fast rotating star Achernar - Thermal infrared interferometry with VLTI/MIDI and SIMECA modeling
• The impact of the oblateness of Regulus on the motion of its companion
• VLTI/PIONIER images the Achernar disk swell
• Massive stellar models: rotational evolution, metallicity effects
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• CoronaGraph Instrument Reference stars for Exoplanets (CorGI-REx) I. Preliminary Vetting and Implications for the Roman Coronagraph and Habitable Worlds Observatory