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We have used mid-infrared long-baseline interferometry with MIDI at the VLTIto resolve the circumstellar material around the Herbig Ae star HR 5999,providing the first direct measurement of its angular size, and to deriveconstraints on the spatial distribution of the dust. A set of ten spectrallydispersed (8-13 micron) interferometric measurements of HR 5999 was obtained.The characteristic size of the emission region depends on the projectedbaseline length and position angle, and it ranges between ~ 5-15milliarcseconds (Gauss FWHM), corresponding to remarkably small physical sizesof ~ 1-3 AU. To derive constraints on the geometrical distribution of the dust,we compared our interferometric measurements to 2D, frequency-dependentradiation transfer simulations of circumstellar disks and envelopes. For diskmodels with radial power-law density distributions, the relatively weak butvery extended emission from outer disk regions (>~ 3 AU) leads to modelvisibilities that are significantly lower than the observed visibilities,making these models inconsistent with the MIDI data. Disk models in which thedensity is truncated at outer radii of ~ 2-3 AU, on the other hand, providegood agreement with the data. A satisfactory fit to the observed MIDIvisibilities of HR 5999 is found with a model of a geometrically thin disk thatis truncated at 2.6 AU and seen under an inclination angle of 58 degr. Neithermodels of a geometrically thin disk seen nearly edge-on, nor models ofspherical dust shells can achieve agreement between the observed and predictedvisibilities. The reason why the disk is so compact remains unclear; wespeculate that it has been truncated by a close binary companion.Comment: accepted for publication by Astronomy & Astrophysic

A compact dusty disk around the Herbig Ae star HR 5999 resolved with VLTI / MIDI