  We measured the mid-infrared extension of the gaseous disk surrounding seven
Be stars in order to constrain the geometry of their circumstellar environments
and to try to infer physical parameters characterizing these disks. We used the
VLTI/MIDI instrument with baselines up to 130 m to obtain an angular resolution
of about 15 mas in the N band and compared our results with previous K band
measurements obtained with the VLTI/AMBER instrument and/or the CHARA
interferometer. We obtained one calibrated visibility measurement for each of
the four stars, p Car, zeta Tau, kappa CMa, and alpha Col, two for delta Cen
and beta CMi, and three for alpha Ara. Almost all targets remain unresolved
even with the largest VLTI baseline of 130m, evidence that their circumstellar
disk extension is less than 10 mas. The only exception is alpha Ara, which is
clearly resolved and well-fitted by an elliptical envelope with a major axis
a=5.8+-0.8mas and an axis ratio a/b=2.4+-1 at 8 microns. This extension is
similar to the size and flattening measured with the VLTI/AMBER instrument in
the K band at 2 microns. The size of the circumstellar envelopes for these
classical Be stars does not seem to vary strongly on the observed wavelength
between 8 and 12microns. Moreover, the size and shape of Alpha Ara's disk is
almost identical at 2, 8, and 12microns.
