2005A&A...437..525P

Isolated Herbig Ae/Be stars are believed to be a class of objects at an intermediate stage between young stellar objects surrounded by massive, optically thick, gaseous and dusty disks and Vega like stars surrounded by debris disks. The Herbig Ae star AB Aurigae is already known for being surrounded by an intermediate-stage dust disk emitting a fairly large infrared and (sub-)millimetric excess. Until now, its outer disk structure has only been resolved at millimeter wavelengths and at optical and near infrared wavelengths with coronographic imaging. We have obtained 20.5µm images which show an unexpected elliptical ring-like emission structure in the disk around AB Aurigae at a distance of about 280AU from the central star. This structure is characterized by a large azimuthal asymmetry in its brightness profile and an off-centered position with respect to the central star. To explain the observations, we propose a simple, purely geometrical model based on an emission ring with an uniform surface brightness, but having an intrinsic eccentricity. Our modeling of this ring-like structure provides valuable constraints on the inclination and the dust composition of the disk. Given the large distance from the central star, only transient heating of very small particles can explain the occurrence of the bright emission ring at mid-infrared wavelengths. Our observations point towards an unexpected geometry of the pre-main-sequence disk. In contrast to the usual sketch of a disk having a constant flaring angle, we argue that the circumstellar disk has a sudden, non uniform increase in the disk thickness. This sudden increase in the disk thickness as inferred by our modeling could be caused by disk instabilities. This suggests, together with the derived intrinsic eccentricity of the emission ring, the presence of a still undetected massive planetary type object, in an orbit at the outer-parts of the disk, disturbing the disk structure through gravitational interaction.