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Isolated HAEBE stars are believed to represent an intermediate stage ofobjects between young stellar objects surrounded by massive, optically thick,gaseous and dusty disks and Vega like stars surrounded by debris disks. Thestar AB Aur is already known for being surrounded by an intermediate-stage dustdisk emitting a fairly large infrared and (sub-)millimetric excess. Until now,the outer disk structure has only been resolved at millimeter wavelengths andat optical wavelength coronographic imaging. We have obtained 20 microns imageswhich show an unexpected ellipse-shaped disk structure in emission at adistance of about 260 AU from the central star. Large azimuthal asymmetries inbrightness can be noticed and the center of the ellipse does not coincide withthe star. A simple, pure geometrical model based on an emission ring of uniformsurface brightness, but having an intrinsic eccentricity succeeds in fittingthe observations. These observations give for the first time clues on a verypeculiar structure of pre-main-sequence disk geometry, i.e. a non uniformincrease in the disk thickness unlike the common usual sketch of a disk with aconstant flaring angle. They provide also valuable informations on the diskinclination as well as its dust composition; at such a large distance from thestar, only transient heating of very small particles can explain such a brightring of emission at mid-infrared wavelengths. Finally, the increase ofthickness inferred by the model could be caused by disk instabilities; theintrinsic eccentricity of the structure might be a clue to the presence of amassive body undetected yet

An emission ring at 20.5 $\mathsf{\mu}$m around the HAEBE star AB Aurigæ: Unveiling the disk structure