Letter to the Editor

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

¹ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France e-mail: Pierre.Kervella@obspm.fr
² Lab. H. Fizeau, CNRS UMR 6525, Univ. de Nice-Sophia Antipolis, Observatoire de la Côte d'Azur, 06108 Nice Cedex 2, France

Received: 11 March 2008
Accepted: 16 April 2008

Abstract

Context. The Be stars are massive dwarf or subgiant stars that present temporary emission lines in their spectrum, and particularly in the Hα 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, A&A, 474, L49) 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 there are not enough data to derive the orbital parameters, they indicate a period of about 15 yr. The projected angular separation of the two objects in December 2007 was less than 0.15″, or 6.7 AU at the distance of Achernar.

Conclusions. From its flux distribution in the near- and thermal-infared, 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.