Context :

Aims : We have used mid-infrared long-baseline interferometry 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 .

Methods : MIDI at the VLTI was used to obtain a set of ten spectrally dispersed ( 8 - 13 \mu m ) interferometric measurements of HR 5999 at different projected baseline lengths and position angles .
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 .

Results : The derived visibility values between \sim 0.5 and \sim 0.9 show that the mid-infrared emission from HR 5999 is clearly resolved .
The characteristic size of the emission region depends on the projected baseline length and position angle , and it ranges between \sim 5 - 15 milliarcseconds ( Gauss FWHM ) , corresponding to remarkably small physical sizes of \sim 1 - 3 AU .
For disk models with radial power-law density distributions , the relatively weak but very extended emission from outer disk regions ( \ga 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 \sim 2 - 3 AU , on the other hand , provide good agreement with the data .

Conclusions : 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 ( i.e .
closer to an edge-on view than to a face-on view ) .
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 .