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

Abstract

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 µm) 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 ∼0.5 and ∼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 ∼5−15 milliarcsec (Gauss FWHM), corresponding to remarkably small physical sizes of ∼1−3 AU. 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. 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◦ (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.