VLTI MIDI observations of the Herbig Ae star HR 5999

We have used long-baseline mid-infrared interferometric observations with MIDI at the VLTI to resolve the circumstellar material around the Herbig Ae star HR 5999 and to provide the first direct measurement of its angular size. The obtained visibilities range between similar to 0.5 and similar to 0.9 at 10 mu m. This implies that the mid-infrared emission from HR 5999 is clearly resolved, but has a compact structure not much larger than the resolution limit of the MIDI observations. The characteristic size of the emission region depends on the position angles, and ranges between 10 and 25 milli-arcseconds (corresponding to similar to 2 similar to 5 AU) for a uniform-disk model, and 5 - 15 mas (similar to 1 similar to 3 AU) for a Gauss model (FWHM). We find a dependence of the characteristic size on the projected baseline position angle, which suggests asymmetry of the emission region. To derive constraints on the geometry of the dust distribution, we compare our interferometric measurements to 2D, frequency-dependent radiation transfer simulations of circumstellar disks.