  The optically invisible infrared-source NGC 2264 IRS 1 is thought to be a
massive young stellar object (~10 Msun). Although strong infrared excess
clearly shows that the central object is surrounded by large amounts of
circumstellar material, no information about the spatial distribution of this
circumstellar material has been available until now. We used the ESO Very Large
Telescope Interferometer to perform long-baseline interferometric observations
of NGC 2264 IRS 1 in the mid-infrared regime. Our observations resolve the
circumstellar material around NGC 2264 IRS 1, provide the first direct
measurement of the angular size of the mid-infrared emission, and yield direct
constraints on the spatial distribution of the dust. We use different
approaches (a geometrical model, a temperature-gradient model, and radiative
transfer models) to jointly model the observed interferometric visibilities and
the spectral energy distribution. The derived visibility values between ~0.02
and ~0.3 show that the mid-infrared emission is clearly resolved. The
characteristic size of the MIR-emission region is ~30-60 AU; this value is
typical for other YSOs with similar or somewhat lower luminosities. A
comparison of the sizes for the two position angles shows a significant
elongation of the dust distribution. Simple spherical envelope models are
therefore inconsistent with the data. The radiative transfer modeling of our
data suggests that we observe a geometrically thin and optically thick
circumstellar disk with a mass of about 0.1 Msun. Our modeling indicates that
NGC 2264 IRS 1 is surrounded by a flat circumstellar disk that has properties
similar to disks typically found around lower-mass young stellar objects. This
result supports the assumption that massive young stellar objects form via
accretion from circumstellar disks.
