PAH luminosity (L_PAH/L_∗) of a sample of Herbig stars compared to the MIR spectral index (F₃₀/F_13.5). The MIR spectral index can be used as a tracer of disk gaps, as indicated by the symbols showing the flaring/transitional disks (group I) with silicate features (red circles), without silicate features (orange triangles), and self-shadowed disks (group II) with silicate features (blue squares). While the PAH luminosities among the objects span ~2–3 orders of magnitude, the averaged luminosities of the disk groups only differ by a factor of ~2–3. The parameters of all the objects are listed in Table 1.