We investigate the nature of the innermost regions of seven circumstellar disks around pre-main-sequence stars. Our object sample contains disks apparently at various stages of their evolution. Both single stars and spatially resolved binaries are considered. In particular, we search for inner disk gaps as proposed for several young stellar objects. When analyzing the underlying dust population in the atmosphere of circumstellar disks, the shape of the 10um feature is investigated. We performed interferometric observations in N band 8-13um with MIDI using baseline lengths of between 54m and 127m. The data analysis is based on radiative-transfer simulations using the Monte Carlo code MC3D by modeling simultaneously the SED, N band spectra, and interferometric visibilities. Correlated and uncorrelated N band spectra are compared to investigate the radial distribution of the dust composition of the disk atmosphere. Spatially resolved mid-infrared emission was detected in all objects. For four objects, the observed N band visibilities and corresponding SEDs could be simultaneously simulated using a parameterized active disk-model. For the more evolved objects of our sample, a purely passive disk-model provides the closest fit. The visibilities inferred for one source allow the presence of an inner disk gap. For another object, one of two visibility measurements could not be simulated by our modeling approach. All uncorrelated spectra reveal the 10um silicate emission feature. In contrast to this, some correlated spectra of the observations of the more evolved objects do not show this feature, indicating a lack of small silicates in the inner versus the outer regions of these disks. We conclude from this observational result that more evolved dust grains can be found in the more central disk regions.
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