Letter to the Editor

The environment of the fast rotating star Achernar ^*,**

II. Thermal infrared interferometry with VLTI/MIDI

¹ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Univ. Paris Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, 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
³ Max-Planck-Intitut für Radioastronomie, Auf dem Hugel 69, 53121 Bonn, Germany

Received: 17 September 2008
Accepted: 10 December 2008

Abstract

Context. As is the case of several other Be stars, Achernar is surrounded by an envelope, recently detected by near-IR interferometry.

Aims. We search for the signature of circumstellar emission at distances of a few stellar radii from Achernar, in the thermal IR domain.

Methods. We obtained interferometric observations on three VLTI baselines in the N band (8-13 μm), using the MIDI instrument.

Results. From the measured visibilities, we derive the angular extension and flux contribution of the N band circumstellar emission in the polar direction of Achernar. The interferometrically resolved polar envelope contributes 13.4 ± 2.5% of the photospheric flux in the N band, with a full width at half maximum of 9.9 ± 2.3 mas (≈). This flux contribution is in good agreement with the photometric IR excess of 10-20% measured by fitting the spectral energy distribution. Due to our limited azimuth coverage, we can only establish an upper limit of 5-10% for the equatorial envelope. We compare the observed properties of the envelope with an existing model of this star computed with the SIMECA code.

Conclusions. The observed extended emission in the thermal IR along the polar direction of Achernar is well reproduced by the existing SIMECA model. Already detected at 2.2 μm, this polar envelope is most probably an observational signature of the fast wind ejected by the hot polar caps of the star.