The structure of the protoplanetary disk surrounding three young intermediate mass stars ^*

II. Spatially resolved dust and gas distribution

¹ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: dfedele@mpia.de
² Dipartimento di Astronomia, Universitá degli studi di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
³ European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching bei München, Germany
⁴ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
⁵ Sterrenkundig Instituut “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁶ Astrophysikalisches Institut Potsdam, 14482 Potsdam, Germany

Received: 5 May 2008
Accepted: 3 September 2008

Abstract

Aims. We present the first direct comparison of the distribution of the gas, as traced by the [O I] 6300 Å emission, and the dust, as traced by the 10 μm 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 Å [O I] line of the same targets.

Results. HD 101412 and HD 135344 B show compact (<2 AU) 10 μm 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 μm emission with the first ring peaking at ~1 AU and the second at ~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 ~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 μm emission emerges from the inner rim and from the flared surface of the disk at larger radii. No dust emission is detected between ~3–15 AU. The oxygen emission seems also to come from a flared structure, however, the bulk of this emission is produced between ~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.