We present the first VLTI/MIDI observations of the Be star \alpha Ara , 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 \degr and 55 \degr , respectively .
These measurements put an upper limit to the envelope size in the N band under the Uniform disk approximation of \phi _ { max } = 4 \pm 1.5 mas , corresponding to 14 R _ { \star } , assuming R _ { \star } =4.8 { R } _ { \odot } 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 ( ( 1995 ) ) and Stee & Bittar ( ( 2001 ) ) .
Optical spectra taken with the échelle instrument Heros and the ESO-50cm telescope , as well as infrared ones from the 1.6m Brazilian telescope have been used together with the MIDI spectra and visibilities .
These observations put complementary constraints on the density and geometry of \alpha Ara circumstellar disk .
We discuss on the potential truncation of the disk by a companion and we present spectroscopic indications of a periodic perturbation of some Balmer lines .