Christoph Leinert

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.

Resolving the innermost parsec of Centaurus A at mid-infrared wavelengths ?

Context. To reveal the origin of mid-infrared radiation from the core of Centaurus A, we carried out interferometric observations with the MID-infrared Interferometer (MIDI) at ESO’s VLTI telescope array. Aims. Observations were obtained with four baselines between unit telescopes of the VLTI, two of them roughly along the radio axis and two orthogonal to it. The interferometric measurements are spectrally resolved with λ/∆λ = 30 in the wavelength range 8 to 13 µm. Their resolution reaches 15 mas at the shortest wavelengths. Supplementary observations were obtained in the near-infrared with the adaptive optics instrument NACO, and at mm wavelengths with SEST and JCMT. Methods. The mid-infrared emission from the core of Centaurus A is dominated by an unresolved point source (<10 mas). Observations with baselines orientated perpendicular to the radio jet reveal an extended component which can be interpreted as a geometrically thin, dusty disk, the axis of which is aligned with the radio jet. Its diameter is about 0.6 pc. It contributes between 20% (at λ � 8 µm) and 40% (at λ � 13 µm) to the nuclear flux from Centaurus A and contains dust at about 240 K. We argue, that the unresolved emission is dominated by a synchrotron source. Its overall spectrum is characterized by an Fν ∼ ν −0.36 power-law which cuts off exponentially towards high frequencies at νc = 8 × 10 13 Hz and becomes optically thick at ν<ν 1 � 45 GHz. Results. Based on a Synchrotron Self Compton (SSC) interpretation for the γ-ray emission, we find a magnetic field strength of 26 µT and a maximum energy of relativistic electrons of γc = Ec/mec 2 = 8500. Near γc, the acceleration time scale is τacc = 4 days, in good agreement with the fastest flux variations, observed at X-ray frequencies. Our SSC model argues for a Doppler factor δ � 1w hich – together with the jet-counter jet ratio of the radio jets on parsec scale – results in an upper limit for the bulk Lorentz factor Γjet < 2.5, at variance with the concept of a “mis-directed BL Lac object”. Conclusions. We estimate a thermal luminosity of the core, Pth � 1.3 × 10 34 W = 1.5 × 10 −4 × LEdd, intermediate between the values for highly efficiently accreting AGN (e.g. Seyfert galaxies) and those of typical FR I radio galaxies. This luminosity, which is predominantly released in X-rays, is most likely generated in an Advection Dominated Accretion Flow (ADAF) and seems just sufficient to heat the dusty disk.

Zodiacal light — A measure of the interplanetary environment

This paper reviews research related to zodiacal light and tries to give the status for the end of 1974.

Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey

Aims. The emission of warm dust dominates the mid-infrared spectra of active galactic nuclei (AGN). Only interferometric observations provide the necessary angular resolution to resolve the nuclear dust and to study its distribution and properties. The investigation of dust in AGN cores is hence one of the main science goals for the MID-infrared Interferometric instrument MIDI at the VLTI. As the first step, the feasibility of AGN observations was verified and the most promising sources for detailed studies were identified. Methods. This was carried out in a “snapshot survey” with MIDI using Guaranteed Time Observations. In the survey, observations were attempted for 13 of the brightest AGN in the mid-infrared which are visible from Paranal. Results. The results of the three brightest, best studied sources have been published in separate papers. Here we present the interferometric observations for the remaining 10, fainter AGN. For 8 of these, interferometric measurements could be carried out. Size estimates or limits on the spatial extent of the AGN-heated dust were derived from the interferometric data of 7 AGN. These indicate that the dust distributions are compact, with sizes on the order of a few parsec. The derived sizes roughly scale with the square root of the luminosity in the mid-infrared, s ∝ √ LMIR, with no clear distinction between type 1 and type 2 objects. This is in agreement with a model of nearly optically thick dust structures heated to T ∼ 300 K. For three sources, the 10 μm feature due to silicates is tentatively detected either in emission or in absorption. Conclusions. The faint AGN of the snapshot survey are at the sensitivity limit of observations with MIDI. Thus, the data set presented here provides a good insight into the observational difficulties and their implications for the observing strategy and data analysis. Based on the results for all AGN studied with MIDI so far, we conclude that in the mid-infrared the differences between individual galactic nuclei are greater than the generic differences between type 1 and type 2 objects.

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.

FU Orionis: The MIDI VLTI Perspective*

We present the first mid-infrared interferometric measurements of FU Orionis. We clearly resolve structures that are best explained with an optically thick accretion disk. A simple accretion disk model fits the observed SED and visibilities reasonably well and does not require the presence of any additional structure such as a dusty envelope. The inclination and also the position angle of the disk can be constrained from the multibaseline interferometric observations. Our disk model is in general agreement with most published near-infrared interferometric measurements. From the shape and strength of the 8-13 μm spectrum, the dust composition of the accretion disk is derived for the first time. We conclude that most dust particles are amorphous and already much larger than those typically observed in the ISM. Although the high accretion rate of the system provides both high temperatures out to large radii and an effective transport mechanism to distribute crystalline grains, we do not see any evidence for crystalline silicates in either the total spectrum or the correlated flux spectra from the inner disk regions. Possible reasons for this nondetection are mentioned. All results are discussed in context with other high spatial resolution observations of FU Ori and other FU Ori objects. We also address the question whether FU Ori is in a younger evolutionary stage than a classical T Tauri star.

First VLTI/MIDI observations of a Be star: Alpha Arae

We present the first VLTI/MIDI observations of the Be star alpha Ara (HD 158 427), showing a nearly unresolved circumstellar disk in the N band. The interferometric measurements made use of the UT1 and UT3 telescopes. The projected baselines were 102 and 74 meters with position angles of 7 ° and 55°, respectively. These measurements put an upper limit on the envelope size in the N band under the uniform disk approximation of φmax= 4±1.5 mas, corresponding to 14 R*, assuming R*=4.8 Rȯ and the Hipparcos distance of 74 pc. On the other hand the disk density must be large enough to produce the observed strong Balmer line emission. In order to estimate the possible circumstellar and stellar parameters we have used the SIMECA code developed by Stee et al. (1995, A&A, 300, 219) and Stee & Bittar (2001, A&A, 367, 532). Optical spectra taken with the echelle instrument Heros and the ESO-50 cm telescope, as well as infrared ones from the 1.6m Brazilian telescope were used together with the MIDI spectra and visibilities. These observations place complementary constraints on the density and geometry of the alpha Ara circumstellar disk. We discuss the potential truncation of the disk by a companion and we present spectroscopic indications of a periodic perturbation of some Balmer lines.