  We present high sensitivity sub-arcsecond resolution images of the Herbig Ae
star AB Aurigae at 11.6 and 18.5 microns taken with Michelle on Gemini North.
Bright extended dust emission close to the star is resolved at both
wavelengths, with quadratically subtracted FWHM of 17+/-4 AU at 11.6 microns
and 22+/-5 AU at 18.5 microns. Additional, fainter emission is detected out to
a radius of 280 AU at 11.6 microns and 350 AU at 18.5 microns down to the
sensitivity limit of the observations. The latter value is identical to the
measured size of the millimeter-continuum disk, but much smaller than the CO
disk. Assuming moderately absorbing material, we find that larger particles (~
1 microns) dominate the mid-IR emission in the inner (< 100 AU) regions of the
disk, and smaller particles (< 0.3 microns) dominate in the outer regions of
the disk. A model of a nearly face-on passive flared disk with an inner rim
accounts well for our observations.
