2005A&A...435..275C

We present the first VLTI/MIDI observations of the Be star alpha Ara (HD158427), 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 φ_max=4±1.5mas, corresponding to 14R_*, assuming R_*=4.8R_☉ and the Hipparcos distance of 74pc. 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. (1995A&A...300..219S) and Stee & Bittar (2001A&A...367..532S). Optical spectra taken with the echelle 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 alphaAra 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.