Three baselines of MIDI data overplotted (at a single continuum wavelength, indicated in cyan) with all double power-law MCMax models computed in Sect. 5. The dotted blue line, “Best 1” in Table 1, is the best single power-law MCMax model in terms of fitting the near-IR visibilities (see Figs. 4 and 5). The dashed red line corresponds to the best-fitting double power-law model with the metallic Fe opacity source (“Best 2” in Table 1). The full green line shows the best-fitting double power-law model when metallic Fe is replaced by amorphous C (“Best alt.” in Table 1). See Sects. 4.2, 5.1, and 6 for a discussion of the models. The inset shows the data at the 30 m baselines as a function of wavelength, with the same double power-law models overplotted as in the main panel.