Context: It is hypothesized that low-mass young stellar objects undergo eruptive phases during their early evolution. The outburst of V1647 Ori between 2003 and 2006 offered a rare opportunity to investigate such an accretion event. Aims: By means of our interferometry observing campaign during this outburst, supplemented by other observations, we investigate the temporal evolution of the inner circumstellar structure of V1647 Ori We also study the role of the changing extinction in the brightening of the object and separate it from the accretional brightening. Methods: We observed V1647 Ori with MIDI/VLTI at two epochs in this outburst. First, during the slowly fading plateau phase (2005 March) and second, just before the rapid fading of the object (2005 September), which ended the outburst. We used the radiative transfer code MC3D to fit the interferometry data and the spectral energy distributions from five different epochs at different stages of the outburst. The comparison of these models allowed us to trace structural changes in the system on AU-scales. We also considered qualitative alternatives for the interpretation of our data. Results: We found that the disk and the envelope are similar to those of non-eruptive young stars and that the accretion rate varied during the outburst. We also found evidence for the increase of the inner radii of the circumstellar disk and envelope at the beginning of the outburst. Furthermore, the change of the interferometric visibilities indicates structural changes in the circumstellar material. We test a few scenarios to interpret these data. We also speculate that the changes are caused by the fading of the central source, which is not immediately followed by the fading of the outer regions. However, if the delay in the fading of the disk is responsible for the changes seen in the MIDI data, the effect should be confirmed by dynamical modeling.

Low-mass young stellar objects (YSOs) show variability not just at optical and near-infrared wavelengths, but also in the mid-infrared. The amplitude and time-scale of the brightness variations depend on the underlying physical mechanisms (e.g., dust processing, grain growth) and the structure of the circumstellar material. Variability studies can therefore provide us information on the structure and evolution of circumstellar material and can help to understand the initial conditions for planet formation. We studied the structure of the circumstellar material at a several AU scale around two low-mass YSOs, V1647 Ori and DG Tau. For both objects, we obtained multi-epoch mid-infrared interferometric observations with MIDI (Leinert et al. 2003), the mid-infrared instrument on the Very Large Telescope Interferometer (ESO/VLTI). V1647 Ori Background Although many young stars exhibit temporal brightenings possibly caused by fluctuating accretion, the eruptions of the FU Orionisand EX Lupi-type classes of variable stars (FUors and EXors) may represent the most intense bursts (e.g., Hartmann & Kenyon 1996). Such outbursts are characterized by an optical brightening of 2-6 magnitude. FUor eruptions can last from some years to several decades, while EXor-outbursts are usually considered as shorter counterparts of those of FUors (i.e., some months long). The triggering mechanism (thermal and/or gravitational instability) and the difference between the physics of FUor and EXor-outbursts are unclear. It is hypothesized that all low-mass premain sequence stars undergo FUor and/or EXor phases. Thus understanding the physics of the outbursts will shed light on fundamental processes of the early evolution of Sun-like stars and their circumstellar disks (structure, dust composition) and in turn on the changes of the conditions in the disks that govern the formation of planetary systems. One of the bestdocumented and most-studied outbursts in the history of eruptive low-mass young stars is that of V1647 Ori in 2003-2006 (see the references collected by Aspin & Reipurth 2009). V1647 Ori was observed with MIDI at two epochs during its 2003-2006 outburst to investigate the temporal evolution of its circumstellar structure and physical processes related to the eruption (Mosoni et al. 2013). Modeling Optical-infrared SEDs at five epochs were compiled. For modeling we used the MC3D radiative transfer code (Wolf et al. 1999, Schegerer et al. 2008). First, we created a reference model for the 2005 March, and later on modified it to fit the data of other epochs. Our modeling, with a smoothly decreasing accretion rate as a major varying parameter, provided good fits of the SEDs at different stages of the outburst. It is important to note that the inner radii of the dust disk and envelope also had to increase during the transition from quiescence to the outburst peak. The latter finding is clear evidence of dynamical variations in the inner circumstellar environment of V1647 Ori. This dust clearing is likely caused by the evaporation of the dust grains due to the outburst heat. VLTI/MIDI data obtained during both the slow and rapid fading stages indicate a considerable change of the circumstellar structure. In constrast to our expectations, based on our model sequence, the object looked more resolved at the second epoch. Our attempts of changing the disk parameters failed the reproduce the data.The increase of the inner radius of the dusty envelope, i.e., rapid removal of dust from the inner 3 AU of the envelope can be due to wind or outflow processes. Two possible scenarios are: 1. A blown-up sperical cavity scenario is produced at the end of the outburst phase. 2. A warm dust halo disappeared by 2005 September. The cavity was created at the beginning of the outburst and was continuously filled with dust, but was cleared as the wind started to wane. We may also see V1647 Ori in a non-equilibrium situation in 2005 September, when the sudden fading of the central source was not yet followed by the fading of the optically thick circumstellar material. The exact study of such an event would require time-dependent radiative transfer modeling. Fig. 1. Light curves of V1647 Ori. Epochs of MIDI observations are indicated. Fig. 2. Schematic picture of the model geometry of the object (not to scale). The line of sight crosses through the envelope. The inclination of the disk is ~60 degree, the opening angle of the conical cavity is ~50 degree. Parameters of the best model for 2005 March reference epoch. Fitted parameters shown in italics. Model parameters changed for different epochs. Note the change of the inner dust radius between quiescence and outburst. During the outburst the accretion rate decreased continuously. The inner radius of the envelope changed between the epochs of MIDI observations. Fig. 3. Spectral energy distributions of V1647 Ori at five different epochs. Between the outburst epochs only the accretion rate was changed here. Note that the model for 2005 September shown here does not fit the MIDI data. The last panel shows all models. The decrease of the brightness of the object with time, from peak (top) to quiescence (bottom) is continuous. Fig. 4. Fits of SEDs and MIDI data. In the bottom left panel the solid line, in the bottom right panel the open triangles show the model where only the accretion rate was decreased relative to the reference model. The dash-dotted line (bottom left panel) and the filled triangles (bottom middle panel) show the model where the inner radius of the envelope was also increased. In general, our modeling showed that the circumstellar environment of V1647 Ori can be described by a disk and envelope system with parameters that are typical for embedded low-mass YSOs. This finding supports the hypothesis that eruptive YSOs are not peculiar objects, but represent a phase in the evolution of all low-mass YSOs. DG Tau The 10 micron silicate feature is known to show temporal variations in DG Tau. The change could be caused by the variation of the height of the inner rim of the disk. Such variation, via the changing shadowing, should affect the silicate emission of the disk outwards the rim. Dust lifted above the disk by wind can also cause changes, i.e., the silicate emission increases in the inner and decreases in the outer regions. MIDI data allows us to study the mid-infrared emission of DG Tau at different spatial scales, i.e., providing spectra of the inner and outer disk regions. The border between inner and outer regions is defined by the resolution of the observations, i.e., the length of the baselines. DG Tau was observed with MIDI on UT1-UT2 (U12) and UT1-UT3 (U13) baselines with resolutions of a few AUs, at five epochs. Three kinds of spectra were obtained: that of the whole system (total, top left), the inner disk (correlated, top right) and the outer disk (uncorrelated, bottom left).The spectra show slight variations but no strong silicate feature. The total and uncorrelated (outer disk) spectra show weak emission, the correlated (inner disk) spectra absorption. This is consistent with a typical disk system. Variations might be proven by detailed data analysis. However, the continuum subtraction will be difficult. The best quality spectra are shown in the bottom right panel. Errors are shown only at a few wavelength values for clarity. The errors are typically twice higher at the long wavelength end of the N-band in all cases. References: Aspin & Reipurth, 2009, AJ, 138, 1137 ● Bary et al., 2009, ApJ, 706, L68 ● Hartmann & Kenyon, 1996, ARA&A, 34, 207 ● Juhász et al., 2012, ApJ, 744, 118 ● Leinert et al., 2003, ESO Messenger, 112, 13 ● Mosoni et al., 2013, A&A, 552, A62 ● Schegerer et al., 2008, A&A, 478, 779 ● Wolf et al., 1999, A&A, 349, 839 Acknowledgements: The authors thank the ESO/VLTI staff for executing the observations in service mode, Timea Csengeri for obtaining photometry at IAC, Aurora Sicilia-Aguilar and Kees Dullemond for useful discussion, Rainer Köhler for helping with the MIDI data reduction. The research leading to these results has received funding from the European Community's Seventh Framework Programme under Grant Agreement 226604. LM and NS are thankful for the support of the Fizeau Exchange Visitor Program through the European Interferometry Initiative (EII) and OPTICON (an EU funded framework program, contract number RII3-CT-2004-001566). Financial support from the Hungarian OTKA grants K81966, K101393 and NN102014 are acknowledged.

We investigate the structure of the innermost region of three circumstellar disks around pre-main sequence stars HD 142666, AS 205 N, and AS 205 S. We determine the inner radii of the dust disks and, in particular, search for transition objects where dust has been depleted and inner disk gaps have formed at radii of a few tenths of AU up to several AU. We performed interferometric observations with IOTA, AMBER, and MIDI in the infrared wavelength ranges 1.6-2.5um and 8-13um with projected baseline lengths between 25m and 102m. The data analysis was based on radiative transfer simulations in 3D models of young stellar objects (YSOs) to reproduce the spectral energy distribution and the interferometric visibilities simultaneously. Accretion effects and disk gaps could be considered in the modeling approach. Results from previous studies restricted the parameter space. The objects of this study were spatially resolved in the infrared wavelength range using the interferometers. Based on these observations, a disk gap could be found for the source HD 142666 that classifies it as transition object. There is a disk hole up to a radius of R_in=0.30AU and a (dust-free) ring between 0.35AU and 0.80AU in the disk of HD 142666. The classification of AS 205 as a system of classical T Tauri stars could be confirmed using the canonical model approach, i. e., there are no hints of disk gaps in our observations.

Novel geometrical designs of computed tomography (CT) scanners in combination with novel image reconstruction algorithms promise to reduce ionizing radiation exposure to the patient in CT scans. While the sampling density of the Field Of View (FOV) is retained, the image quality can even be increased in contrast to conventional CT scanners. In this study, we present first images obtained with a novel CT scanner that we developed in our working group. In this open CT system with irradiation within a fan beam, parallel Radon data are directly obtained for image reconstruction using the OPED (Orthogonal Polynomial Expansion on the Disk) algorithm. This algorithm uses Radon data directly, i.e., without any further data processing such as rebinning and interpolation. We experimentally test theoretical predictions for this system by quantifying image quality parameters in comparison with corresponding parameters that are derived from the images of a conventional scanner of the 3(rd) generation. The modulation transfer function (MTF) and noise power spectrum (NPS) are determined using a test phantom. The novel CT system quantitatively shows the same noise property as the conventional scanner. The resolution that is reached in the center of a reconstructed image is nearly identical for both scanner types. But we found that the resolution that is achieved in the novel CT system does not depend on the image position while the MTF of the conventional scanner decreases for radially outer regions of the image.

Aims. We search for frozen water and its processing around young stellar objects (YSOs of class I/II). We try to detect potential, regional differences in water ice evolution within YSOs, which is relevant to understanding the chemical structure of the progenitors of protoplanetary systems and the evolution of solid materials. Water plays an important role as a reaction bed for rich chemistry and is an indispensable requirement for life as known on Earth.Methods. We present our analysis of NAOS-CONICA/VLT spectroscopy of water ice at 3 mu m for the T Tauri star YLW16A in the rho Ophiuchi molecular cloud. We obtained spectra for different regions of the circumstellar environment. The observed absorption profiles are deconvolved with the mass extinction profiles of amorphous and crystallized ice measured in laboratory. We take into account both absorption and scattering by ice grains.Results. Water ice in YLW16A is detected with optical depths of between tau = 1.8 and tau = 2.5. The profiles that are measured can be fitted predominantly by the extinction profiles of small grains (0.1 mu m-0.3 mu m) with a small contribution from large grains (<10%). However, an unambiguous trace of grain growth cannot be found. We detected crystallized water ice spectra that have their origin in different regions of the circumstellar environment of the T Tauri star YLW16A. The crystallinity increases in the upper layers of the circumstellar disk, while only amorphous grains exist in the bipolar envelope. As in studies of silicate grains in T Tauri objects, the higher crystallinity in the upper layers of the outer disk regions implies that water ice crystallizes and remains crystallized close to the disk atmosphere where water ice is shielded against hard irradiation.

Aims: We aim at using the ling baselines of the VLI interferometer and the mid-IR combiner MIDI (8-13 mu m) to derive the morphology of the protoplanetary disk surrounding the Herbig Ae star AB AurigaeMethods. We present the first N-band analysis of AB Aur performed with a maximum angular resolution of 17 mas (2.5 AU at the Taurus-Auriga distance). We used the radiative transfer code MC3D and a silicate-dominated dust grain mixture to fit the spectral energy distribution (SED), together with the N-band dispersed visibilities (lambda/delta lambda similar to 30) and to constrain the inner-disk spatial structure.Results. The silicate band is prominent in the similar to 300 mas FOV of the MIDI instrument, the emission reaches 70 to 90% of the total flux measured by ISO. The circumstellar emission (CSE) is resolved even at the shortest baselines. The spectrally dispersed visibilities show a steep drop between 8 and 9.5 mu m, followed by a plateau between 10 and 13 mu m. Our modelling shows that the observed SED and visibilities can be reproduced with a simple passive disk model. For such a weakly inclined disk (i similar to 30 deg), the mid-IR visibilities can directly determine the flaring index, while the scale height can be subsequently and unambiguously derived from the combination of the spectral and interferometric constraints. The modelling yields typical values for the scale height of about 8 AU at a radial distance of 100 AU and a flaring index in the range 1.25-1.30 for the explored range of model input parameters.Conclusions. The radial structure of the circumstellar inner disk around AB Aur is directly determined by MIDI. The radiative transfer modelling demonstrates the powerful synergy of interferometry and spectro-photometry to alleviate the degeneracy, which may hamper determining the disk morphology. Our analysis supports the classification of AB Aur among the flared disks of the first group in the Meeus classification.

Aims. High-energy irradiation of circumstellar material might impact the structure and the composition of a protoplanetary disk and hence the process of planet formation. In this paper, we present a study of the possible influence of stellar irradiation, indicated by X-ray emission, on the crystalline structure of circumstellar dust.Methods. The dust crystallinity is measured for 42 class II T Tauri stars in the Taurus star-forming region using a decomposition fit of the 10 μm silicate feature, measured with the spitzer IRS instrument. Since the sample includes objects with disks of various evolutionary stages, we further confine the target selection, using the age of the objects as a selection parameter.Results. We correlate the X-ray luminosity and the X-ray hardness of the central object with the crystalline mass fraction of the circumstellar dust and find a significant anti-correlation for 20 objects within an age range of approx. 1 to 4.5 Myr. We postulate that X-rays represent the stellar activity and consequently the energetic ions of the stellar winds which interact with the circumstellar disk. We show that the fluxes around 1 AU and ion energies of the present solar wind are sufficient to amorphize the upper layer of dust grains very efficiently, leading to an observable reduction of the crystalline mass fraction of the circumstellar, sub-micron sized dust. This effect could also erase other relations between crystallinity and disk/star parameters such as age or spectral type.

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 mu m feature should additionally be investigated.Methods. We performed interferometric observations in N band (8-13 mu m) 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, HD72106 and HBC639, 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 mu m 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.

Aims. The aim of this study is to enhance our knowledge of the characteristics and distribution of the circumstellar dust associated with the individual components of the young hierarchical triple system T Tau.Methods. To reach this goal, observations in the N-band (8-13 mu m) with the two-telescope interferometric instrument MIDI at the VLTI were performed. For the northern component of the T Tau system, projected baseline lengths of 43 m, 62 m, and 85 m were used. For the southern binary projected baseline lengths of equivalent resolution could be utilised. Our study is based on both the interferometric and the spectrophotometric measurements and is supplemented by new visual and infrared photometry. Also, the phases were investigated to determine the dominating mid-infrared source in the close southern binary. The data were fit with the help of a sophisticated physical disc model. This model utilises the radiative transfer code MC3D that is based on the Monte-Carlo method.Results. Extended mid-infrared emission is found around all three components of the system. Simultaneous fits to the photometric and interferometric data confirm the picture of an almost face-on circumstellar disc around T Tau N. Towards this star, the silicate band is seen in emission. This emission feature is used to model the dust content of the circumstellar disc. Clear signs of dust processing are found. Towards T Tau S, the silicate band is seen in absorption. This absorption is strongly pronounced towards the infrared companion T Tau Sa as can be seen from the first individual N-band spectra for the two southern components. Our fits support the previous suggestion that an almost edge-on disc is present around T Tau Sa. This disc is thus misaligned with respect to the circumstellar disc around T Tau N. The interferometric data indicate that the disc around T Tau Sa is oriented in the north-south direction, which favours this source as launching site for the east-western jet. We further determine from the interferometric data the relative positions of the components of the southern binary in the N-band. We find good agreement with recent position measurements in the near-infrared.

An innovative method is presented that allows an objective and quantitative determination of quality parameters in computed tomography (CT). In contrast to projection radiography, the image quality parameters of a CT system is not only affected by the performance of the detector alone but additionally by the geometrical alignment of source-object-detector and the combination of numerous single raw exposure frames to one image using a specific reconstruction algorithm. Our scientific objective is to separate the effects of detector and image reconstruction algorithm on the performance of a CT. We focus here on the determination of the Modulation Transfer Function (MTF) and Noise Power Spectrum (NPS) of raw data and CT image.

A new type of X-ray CT scanning geometry is proposed. The geometry of the scanner includes a half ring detector array and resembles the geometry of a scanner of the fourth generation. Unlike the latter, the proposed system collects parallel projections allowing efficient collimation of the incident beam for the purpose of scatter reduction. The geometry of the data collected with the proposed scanner is ideal for algorithms developed for image reconstruction from parallel projections with a non-uniform sampling such as the Orthogonal Polynomial Expansion on Disk (OPED) algorithm. This scanner can be efficiently used in applications where high precision measurements at micrometer scales are required, e.g. in the exact quantification of the morphology of small animals.

MIDI was designed to open the mid-infrared wavelength regime to the VLTI. It allows to resolve and study the warm regions of dusty discs around young stellar objects in great detail due to the perfect match between the spatial resolution provided by the VLTI and the size of the regions dominating in the N-band. The recent years thus consequently lead to new insights into the circumstellar environments of a wide variety of pre-main-sequence stars.

Aims. We study the inner region (∼1.0 AU up to a few 10 AUs) of the circumstellar disk around the “classical” T Tauri star RY Tau. Our aim is to find a physical description satisfying the available interferometric data, obtained with the mid-infrared interferometric instrument at the Very Large Telescope Interferometer, as well as the spectral energy distribution in the visible to millimeter wavelength range. We also compare the findings with the results of similar studies, including those of intermediate-mass Herbig Ae/Be stars. Methods. Our analysis is done within the framework of a passively heated circumstellar disk, which is optionally supplemented by the effects of accretion and an added envelope. To achieve a more consistent and realistic model, we used our continuum transfer code MC3D. In addition, we studied the shape of the 10 µm silicate emission feature in terms of the underlying dust population, both for single-dish and for interferometric measurements. Results. We show that a modestly flaring disk model with accretion can explain both the observed spectral energy distribution and the mid-infrared visibilities obtained with the mid-infrared infrared instrument. We found an interesting ambiguity: a circumstellar active disk model with an added envelope, and a lower accretion rate than in the active disk model without envelope, could represent the observations equally as well. This type of model with the envelope should be considered a viable alternative in future models of other T Tauri stars. The approach of a disk with a puffed-up inner rim wall and the influence of a stellar companion is also discussed. We also investigate the influence of various fit parameters on the outcome of the radiative transfer modeling. From the study of the silicate emission feature we see evidence for dust evolution in a T Tauri star, with a decreasing fraction of small amorphous and an increasing fraction of crystalline particles closer to the star.

We present a spatially resolved 894 μm map of the circumstellar disk of the Butterfly Star in Taurus (IRAS 04302+2247), obtained with the Submillimeter Array (SMA). The predicted and observed radial brightness profiles agree well in the outer disk region but differ in the inner region with an outer radius of ~80-120 AU. In particular, we find a local minimum of the radial brightness distribution at the center, which can be explained by an increasing density/optical depth combined with the decreasing vertical extent of the disk toward the center. Our finding indicates that young circumstellar disks can be optically thick at wavelengths as long as 894 μm. While earlier modeling lead to general conclusions about the global disk structure and, most importantly, evidence for grain growth in the disk (Wolf, Padgett, & Stapelfeldt), the presented SMA observations provide more detailed constraints for the disk structure and dust grain properties in the inner, potentially planet-forming region (≲80-120 AU) versus the outer disk region (~120-300 AU).

We report on observations of circumstellar disks around young stars that have been obtained with the MIDI instrument, which is mounted on the VLT Interferometer and operates in the 10 μm atmospheric window. The maximum spatial resolution of 5 milli-arcsec corresponds to sub-AU scales at the distance to nearby star formation regions. Thus, we can study the disks on the spatial scales at which important processes occur, such as accretion, dust processing, and planet formation. The main results obtained so far can be summarized as follows: 1. The measured interferometric visibilities are in good qualitative agreement with those predicted by models of circumstellar disks. In particular, a predicted correlation between the strength of the far-infrared excess and the spatial structure of the disk is confirmed by direct measurements; 2. In several objects strong evidence for deviations from circular symmetry is present, indicating that an inclined disk is indeed the dominant component seen in the mid-infrared; 3. The dust properties are not uniform over the disk, but are instead a strong function of distance to the central star. The dust in the innermost disk regions is observed to be more "processed" than the dust further out, both in Herbig Ae star disks and in those around T Tauri stars.

Aims. We present a compositional analysis of 8-13 mu m spectra of 32 young stellar objects (YSOs). Our sample consists of 5 intermediate-mass stars and 27 low-mass stars. Although some previous studies give reasons for the similarity between the dust in circumstellar disks of T Tauri stars and Herbig Ae/ Be stars, a quantitative comparison has been lacking so far. Therefore, we include a discussion of the results of the 10 mu m spectroscopic survey of van Boekel et al. (2005, A& A, 437, 189), who focus on Herbig Ae/Be stars, the higher mass counterparts of T Tauri stars, and draw comparisons to this and other studies.Methods. While the spectra of our 32 objects and first scientific results have already been published elsewhere we perform a more detailed analysis of the similar to 10 mu m silicate feature. In our analysis we assume that this emission feature can be represented by a linear superposition of the wavelength-dependent opacity kappa(abs)(lambda) describing the optical properties of silicate grains with different chemical composition, structure, and grain size. Determining an adequate fitting equation is another goal of this study. Using a restricted number of fitting parameters, we investigate which silicate species are necessary for the compositional fitting. Particles, with radii of 0.1 mu m- and 1.5 mu m and consisting of amorphous olivine and pyroxene, forsterite, enstatite, and quartz are considered. Only compact, homogeneous dust grains are used in the presented fitting procedures. In this context we show that acceptable fitting results can also be achieved if emission properties of porous silicate grains are considered instead.Results. Our analysis shows -in terms of the properties of the circumstellar dust-like crystallinity - T Tauri systems are a continuation of HAeBe systems at their lower mass end. However, a weak correlation between grain growth and stellar luminosity could be found, in contrast to HAeBe systems.

We report on observations of circumstellar disks around young stars that have been obtained with the MIDI instrument, which is mounted on the VLT Interferometer and operates in the 10μm atmospheric window. The maximum spatial resolution of 5 milli-arcsec corresponds to sub-AU scales at the distance to nearby star formation regions. Thus, we can study the disks on the spatial scales at which important processes occur, such as accretion, dust processing, and planet formation. The main results obtained so far can be summarized as follows: 1. The measured interferometric visibilities are in good qualitative agreement with those predicted by models of circumstellar disks. In particular, a predicted correlation between the strength of the far-infrared excess and the spatial structure of the disk is confirmed by direct measurements; 2. In several objects strong evidence for deviations from circular symmetry is present, indicating that an inclined disk is indeed the dominant component seen in the mid-infrared; 3. The dust properties are not uniform over the disk, but are instead a strong function of distance to the central star. The dust in the innermost disk regions is observed to be more “processed” than the dust further out, both in Herbig Ae star disks and in those around T Tauri stars.