Identifying gaps in flaring Herbig Ae/Be disks using spatially resolved mid-infrared imaging Are all group I disks transitional?

Context. The evolution of young massive protoplanetary disks toward planetary systems is expected to correspond to structural changes in observational appearance, which includes the formation of gaps and the depletion of dust and gas. Aims. A special group of disks around Herbig Ae/Be stars do not show prominent silicate emission features, although they still bear signs of flaring disks, the presence of gas, and small grains. We focus our attention on four key Herbig Ae/Be stars to understand the structural properties responsible for the absence of silicate feature emission. Methods. We investigate Q- and N-band images taken with Subaru/COMICS, Gemini South /T-ReCS,and VLT/VISIR. We perform radiative transfer modeling to examine the radial distribution of dust and polycyclic aromatic hydrocarbons (PAHs). Our solutions require a separation of inner-and outer-disks by a large gap. From this, we characterize the radial density structure of dust and PAHs in the disk. Results. The inner edge of the outer disk has a high surface brightness and a typical temperature between similar to 100-150 K and therefore, dominates the emission in the Q-band. All four disks are characterized by large gaps. We derive radii of the inner edge of the outer disk of 34(-4)(+4), 23(-5)(+3), 30(-3)(+5) and 63(-4)(+4) AU for HD97048, HD169142, HD135344 B, and Oph IRS 48, respectively. For HD 97048 this is the first detection of a disk gap. The large gaps deplete the entire population of silicate particles with temperatures suitable for prominent mid-infrared feature emission, while small carbonaceous grains and PAHs can still show prominent emission at mid-infrared wavelengths. The continuum emission in the N-band is not due to emission in the wings of PAHs. This continuum emission can be due to very small grains or to thermal emission from the inner disk. We find that PAH emission is not always dominated by PAHs on the surface of the outer disk. Conclusions. The absence of silicate emission features is due to the presence of large gaps in the critical temperature regime. Many, if not all Herbig disks with spectral energy distribution classification group I, are disks with large gaps and can be characterized as (pre-) transitional. An evolutionary path from the observed group I to the observed group II sources seems no longer likely. Instead, both might derive from a common ancestor.