The structure of the protoplanetary disk surrounding three young intermediate mass stars II. Spatially resolved dust and gas distribution

Aims. We present the first direct comparison of the distribution of the gas, as traced by the [O I] 6300 angstrom emission, and the dust, as traced by the 10 mu m emission, in the planet-forming region of proto-planetary disks around three intermediate-mass stars: HD 101412, HD 135344 B and HD 179218. Methods. N-band visibilities were obtained with VLTI/MIDI. Simple geometrical models are used to compare the dust emission to high-resolution optical spectra in the 6300 angstrom [O I] line of the same targets. Results. HD 101412 and HD 135344 B show compact (< 2 AU) 10 mu m emission while the [O I] brightness profile shows a double peaked structure. The inner peak is strongest and is consistent with the location of the dust, the outer peak is fainter and is located at 5-10 AU. In both systems, spatially extended PAH emission is found. HD 179218 shows a double ring-like 10 mu m emission with the first ring peaking at similar to 1 AU and the second at similar to 20 AU. The [O I] emitting region is more compact, peaking between 3-6 AU. Conclusions. The disks around HD 101412 and HD 135344 B appear strongly flared in the gas, but self-shadowed in the dust beyond similar to 2 AU. The difference in the gas and dust vertical structure beyond 2 AU might be the first observational evidence of gas-dust decoupling in protoplanetary disks. The disk around HD 179218 is flared in the dust. The 10 mu m emission emerges from the inner rim and from the flared surface of the disk at larger radii. No dust emission is detected between similar to 3-15 AU. The oxygen emission seems also to come from a flared structure, however, the bulk of this emission is produced between similar to 1-10 AU. This could indicate a lack of gas in the outer disk or could be due to chemical effects which reduce the abundance of OH - the parent molecule of the observed [O I] emission - further away from the star. It may also be a contrast effect if the [O I] emission is much stronger in the inner disk. We suggest that the three systems, HD 179218, HD 135344 B and HD 101412, may form an evolutionary sequence: the disk initially flared becomes flat under the combined action of gas-dust decoupling, grain growth and dust settling.