First VLTI/MIDI observations of a Be star: Alpha Arae

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: Olivier.Chesneau@obs-azur.fr
² Observatoire de la Côte d'Azur, CNRS UMR 6203, Avenue Copernic, Grasse, France
³ Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁴ Laboratoire Universitaire d'Astrophysique de Nice, France
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Instituto de Astronomia, Geofiśica e Ciências Atmosféricas da Universidade de São Paulo (IAG-USP), CP 9638, 01065-970 São Paulo, Brazil
⁷ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 6 September 2004
Accepted: 5 January 2005

Abstract

We present the first VLTI/MIDI observations of the Be star alpha Ara (HD 158 427), showing a nearly unresolved circumstellar disk in the N band. The interferometric measurements made use of the UT1 and UT3 telescopes. The projected baselines were 102 and 74 meters with position angles of 7° and 55°, respectively. These measurements put an upper limit on the envelope size in the N band under the uniform disk approximation of  mas, corresponding to 14 , assuming and the Hipparcos distance of 74 pc.
On the other hand the disk density must be large enough to produce the observed strong Balmer line emission. In order to estimate the possible circumstellar and stellar parameters we have used the SIMECA code developed by Stee et al. (1995, A&A, 300, 219) 
and Stee & Bittar (2001, A&A, 367, 532). Optical spectra taken with the échelle instrument Heros and the ESO-50 cm telescope, as well as infrared ones from the 1.6m Brazilian telescope were used together with the MIDI spectra and visibilities. These observations place complementary constraints on the density and geometry of the alpha Ara circumstellar disk. We discuss the potential truncation of the disk by a companion and we present spectroscopic indications of a periodic perturbation of some Balmer lines.