  We present mid-infrared nulling interferometric and direct imaging
observations of the Herbig Ae star HD 100546 obtained with the Magellan I
(Baade) 6.5 m telescope. The observations show resolved circumstellar emission
at 10.3, 11.7, 12.5, 18.0, and 24.5 microns. Through the nulling observations
(10.3, 11.7 and 12.5 microns), we detect a circumstellar disk, with an
inclination of 45 +- 15 degrees with respect to a face-on disk, a semimajor
axis position angle of 150 +- 10 degrees (E of N), and a spatial extent of
about 25 AU. The direct images (18.0 and 24.5 microns) show evidence for cooler
dust with a spatial extent of 30-40 AU from the star. The direct images also
show evidence for an inclined disk with a similar position angle as the disk
detected by nulling. This morphology is consistent with models in which a
flared circumstellar disk dominates the emission. However, the similarity in
relative disk size we derive for different wavelengths suggests that the disk
may have a large inner gap, possibly cleared out by the formation of a giant
protoplanet. The existence of a protoplanet in the system also provides a
natural explanation for the observed difference between HD 100546 and other
Herbig Ae stars.
