Tracing the potential planet-forming regions around seven pre-main-sequence stars ^*,**

¹ Helmholtz Zentrum München, German Research Center for Environmental Health, Ingolstädter Landstraße 1, 85758 Neuherberg, Germany e-mail: schegerer@mpia-hd.mpg.de
² Max Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
³ University of Kiel, Institute of Theoretical Physics and Astrophysics, Leibnizstraße 15, 24098 Kiel, Germany
⁴ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Straße 2, 85748 Garching, Germany

Received: 11 August 2008
Accepted: 7 April 2009

Abstract

Aims. We investigate the nature of the innermost regions with radii of several AUs of seven circumstellar disks around pre-main-sequence stars, T Tauri stars in particular. 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 (YSOs). When analyzing the underlying dust population in the atmosphere of circumstellar disks, the shape of the 10 μm feature should additionally be investigated.

Methods. We performed interferometric observations in N band (8-13 μm) with the Mid-Infrared Interferometric Instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) using baseline lengths of between 54 m and 127 m. The data analysis is based on radiative-transfer simulations using the Monte Carlo code MC3D by modeling simultaneously the spectral energy distribution (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.

Results. Spatially resolved mid-infrared (MIR) emission was detected in all objects. For four objects (DR Tau, RU  Lup, S CrA N, and S CrA S), 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, HD 72106 and HBC 639, a purely passive disk-model provides the closest fit. The visibilities inferred for the source RU Lup allow the presence of an inner disk gap. For the YSO GW Ori, one of two visibility measurements could not be simulated by our modeling approach. All uncorrelated spectra reveal the 10 μm 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.