  The innermost parts of dusty debris disks around main sequence stars are
currently poorly known due to the high contrast and small angular separation
with their parent stars. Using near-infrared interferometry, we aim to detect
the signature of hot dust around the nearby A4 V star Fomalhaut, which has
already been suggested to harbor a warm dust population in addition to a cold
dust ring located at about 140 AU. Archival data obtained with the VINCI
instrument at the VLTI are used to study the fringe visibility of the Fomalhaut
system at projected baseline lengths ranging from 4 m to 140 m in the K band. A
significant visibility deficit is observed at short baselines with respect to
the expected visibility of the sole stellar photosphere. This is interpreted as
the signature of resolved circumstellar emission, producing a relative flux of
0.88% +/- 0.12% with respect to the stellar photosphere. While our
interferometric data cannot directly constrain the morphology of the excess
emission source, complementary data from the literature allow us to discard an
off-axis point-like object as the source of circumstellar emission. We argue
that the thermal emission from hot dusty grains located within 6 AU from
Fomalhaut is the most plausible explanation for the detected excess. Our study
also provides a revised limb-darkened diameter for Fomalhaut (2.223 +/- 0.022
mas), taking into account the effect of the resolved circumstellar emission.
