  We have used mid-infrared long-baseline interferometry with MIDI at the VLTI
to resolve the circumstellar material around the Herbig Ae star HR 5999,
providing the first direct measurement of its angular size, and to derive
constraints on the spatial distribution of the dust. A set of ten spectrally
dispersed (8-13 micron) interferometric measurements of HR 5999 was obtained.
The characteristic size of the emission region depends on the projected
baseline length and position angle, and it ranges between ~ 5-15
milliarcseconds (Gauss FWHM), corresponding to remarkably small physical sizes
of ~ 1-3 AU. To derive constraints on the geometrical distribution of the dust,
we compared our interferometric measurements to 2D, frequency-dependent
radiation transfer simulations of circumstellar disks and envelopes. For disk
models with radial power-law density distributions, the relatively weak but
very extended emission from outer disk regions (>~ 3 AU) leads to model
visibilities that are significantly lower than the observed visibilities,
making these models inconsistent with the MIDI data. Disk models in which the
density is truncated at outer radii of ~ 2-3 AU, on the other hand, provide
good agreement with the data. A satisfactory fit to the observed MIDI
visibilities of HR 5999 is found with a model of a geometrically thin disk that
is truncated at 2.6 AU and seen under an inclination angle of 58 degr. Neither
models of a geometrically thin disk seen nearly edge-on, nor models of
spherical dust shells can achieve agreement between the observed and predicted
visibilities. The reason why the disk is so compact remains unclear; we
speculate that it has been truncated by a close binary companion.
