Frank Przygodda

The building blocks of planets within the `terrestrial' region of protoplanetary disks

Our Solar System was formed from a cloud of gas and dust. Most of the dust mass is contained in amorphous silicates, yet crystalline silicates are abundant throughout the Solar System, reflecting the thermal and chemical alteration of solids during planet formation. (Even primitive bodies such as comets contain crystalline silicates.) Little is known about the evolution of the dust that forms Earth-like planets. Here we report spatially resolved detections and compositional analyses of these building blocks in the innermost two astronomical units of three proto-planetary disks. We find the dust in these regions to be highly crystallized, more so than any other dust observed in young stars until now. In addition, the outer region of one star has equal amounts of pyroxene and olivine, whereas the inner regions are dominated by olivine. The spectral shape of the inner-disk spectra shows surprising similarity with Solar System comets. Radial-mixing models naturally explain this resemblance as well as the gradient in chemical composition. Our observations imply that silicates crystallize before any terrestrial planets are formed, consistent with the composition of meteorites in the Solar System.

Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI

We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wave- length of 10 µm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 µm to 13.5 µm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 µm are found to be from 1 AU to 10 AU. The 10 µm sizes of our sample stars correlate with the slope of the 10-25 µm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different ge- ometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irra- diated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way.

MIDI - The 10 mu m instrument on the VLTI

After more than five years of preparation, the mid-infrared interferometric instrument MIDI has been transported to Paranal where it will undergo testing and commissioning on theVery Large Telescope Interferometer VLTI from the end of 2002through large part of this year 2003. Thereafter it will be available as a user instrument to perform interferometric observations over the8 μm–13 μm wavelength range, with a spatial resolution of typically 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 3–4 mag or 1–2.5 Jy for self –fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument, laboratory tests, and expected performance, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations.

Evidence for grain growth in T Tauri disks

In this article we present the results from mid-infrared spectroscopy of a sample of 14 T Tauri stars with silicate emission. The qualitative analysis of the spectra reveals a correlation between the strength of the silicate feature and its shape similar to the one which was found recently for the more massive Herbig Ae/Be stars by van Boekel et al. (2003). The comparison with theoretical spectra of amorphous olivine ([Mg,Fe] 2 SiO 4 ) with different grain sizes suggests that this correlation is indicating grain growth in the disks of TTauri stars. Similar mechanisms of grain processing appear to be effective in both groups of young stars.

Ten-micron instrument MIDI: getting ready for observations on the VLTI

The mid-infrared interferometric instrument MIDI is currently undergoing testing in preparation for commissioning on the Very Large Telescope Interferometer VLTI at the end of this year 2002. It will perform interferometric observations over the 8 μm - 13 μm wavelength range, with a spatial resolution of 20 milliarcsec, a spectral resolution of up to 250, and an anticipated point source sensitivity of N = 4 mag or 1 Jy for self-fringe tracking, which will be the only observing mode during the first months of operation. We describe the layout of the instrument and the performance during laboratory tests, both for broadband and spectrally resolved observing modes. We also briefly outline the planned guaranteed time observations.

Analysis of the dust evolution in the circumstellar disks of T Tauri stars

Aims. We present a compositional analysis of 8-13μm 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 TTauri stars and Herbig Ae/Be stars, a quantitative comparison has been lacking so far. Therefore, we include a discussion of the results of the 10μm spectroscopic survey of van Boekel et al. (2005, A&A, 437, 189), who focus on Herbig Ae/Be stars, the higher mass counterparts of TTauri 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 ∼10μm silicate feature. In our analysis we assume that this emission feature can be represented by a linear superposition of the wavelength-dependent opacity K abs (λ) 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 μm- and 1.5μm 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.

Interferometric Observation at MID-Infrared Wave-Lengths with MIDI

MIDI, the MID-Infrared Interferometricnterferometric Instrument for ESOs Very Large Telescope Interferometer (VLTI), will be the first instrument for combining mid-infrared light directly in order to obtain angular resolution up to 10 mas (assuming a 200 m baseline) in a wavelength range from 8 to 13 μm. Currently in the phase of commissioning at Paranal, the start of its scientific operation is expected for summer 2003. Direct interferometry at thermal infrared wavelengths demands special requirements on the instrument and also on the procedures of preparation of data reduction. Hereafter MIDIs different observing modes are described and an example for an interferometric observation is given.

Two-dimensional modulation for holographic data storage systems

Page-oriented holographic data storage systems (HDSSs) generally use spatial light modulators (SLMs) to generate two-dimensional (2D) digital patterns, so-called data pages. These data pages are stored via interference patterns of the object and reference beam in the holographic medium and are retrieved from the medium by exposing it to the reference beam. The reconstructed data pages are then detected by a matrix detector. One important challenge in designing an HDSS is to develop a suitable modulation, which takes into account the specific characteristics of the transfer channel and enables high data capacity, high data transfer rate and low symbol error rate (SER). In this paper, we present a new method for modulating data in an HDSS. The main idea is to adapt the general concept of 2D run-length limited modulation (RLL) to a numerically more efficient implementation usable for HDSS. We demonstrate that this 2D block coding method with a constant weight and a sparse code increases the amount of user data per data page, while the SER remains low compared with standard sparse modulation coding. This results in a higher data rate and higher data density.

Test of key elements for common path holography

Holographic devices are expected to have a much larger capacity than conventional optical storage systems such as CD, DVD or blue diode laser based HD-DVD or BD. Recent developments in the field of dedicated recording materials and advanced optical enabling technologies are now opening the door for the realization of commercial products. One of the major technical challenges is the development of a robust and reliable system concept, which allows easy exchangeability of the medium. We developed a holographic tester system with common paths for the reference and the signal beams based on a single mode blue laser diode and a commercial CMOS detector. The system will be used to evaluate various multiplexing schemes, to investigate the influence of system tolerances on the reading performance and to estimate fundamental system limitations.

Material consumption and crosstalk characteristics of different holographic storage concepts

Holographic data storage is considered to be one of the most promising technologies for high-capacity data storage. Several holographic concepts have been suggested and investigated in detail by many companies. The concepts differ in the method of superposing object and reference beams inside the holographic medium. At present, the most relevant concepts are the plane wave concept, the collinear concept, and a concept with counterpropagating beams. We compare all three concepts, with regard to their beam overlap, efficiency of material consumption, diffraction efficiency, and crosstalk characteristics. The investigation is performed by numerical simulations, which offer well-defined conditions in all setups and are independent of experimental uncertainties such as the nonlinear behavior of medium sensitivity and the effects of light scattering or reflection.