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

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Abstract: Context: Be stars are massive dwarf or subgiant stars that present temporaryemission lines in their spectrum, and particularly in the Halpha line. Themechanism triggering these Be episodes is currently unknown, but binarity couldplay an important role. Aims: Previous observations with the VLT-VISIRinstrument Kervella and Domiciano de Souza 2007 revealed a faint companion toAchernar, the brightest Be star in the sky. The present observations areintended to characterize the physical nature of this object. Methods: Weobtained near-IR images and an H-band spectrum of Achernar B using the VLT-NACOadaptive optics systems. Results: Our images clearly show the displacement ofAchernar B over a portion of its orbit around Achernar A. Although these dataare insuficient to derive the orbital parameters, they indicate a period of theorder of 15 yr. The projected angular separation of the two objects in December2007 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 Bis most likely an A1V-A3V star. Its orbital period appears similar to theobserved pseudo-periodicity of the Be phenomenon of Achernar. This indicatesthat an interaction between A and B at periastron could be the trigger of theBe episodes.

Author: Pierre Kervella LESIA, Armando Domiciano De Souza, Philippe Bendjoya


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