Optical depth variation at different physical radii. Three wavelengths are explored in the panels from top to bottom at 8.5, 10, and 11.5 μm, respectively. The radius is a 1/2 spatial scale probed by the interferometer (see Sect. 4.2) converted to physical units at the adopted distance. The optical depth is color-coded with regard to our parameter, η (see Sect. 4.2). The vertical gray dotted lines in the middle panel mark the upper limit of the radial extent of the HBe disk in the 10.4 μm image (Fig. 4) at 50% and 1% of the normalized peak.