U. Graser

The central dusty torus in the active nucleus of NGC 1068.

Active galactic nuclei (AGNs) display many energetic phenomena—broad emission lines, X-rays, relativistic jets, radio lobes—originating from matter falling onto a supermassive black hole. It is widely accepted that orientation effects play a major role in explaining the observational appearance of AGNs. Seen from certain directions, circum-nuclear dust clouds would block our view of the central powerhouse. Indirect evidence suggests that the dust clouds form a parsec-sized torus-shaped distribution. This explanation, however, remains unproved, as even the largest telescopes have not been able to resolve the dust structures. Here we report interferometric mid-infrared observations that spatially resolve these structures in the galaxy NGC 1068. The observations reveal warm (320 K) dust in a structure 2.1 parsec thick and 3.4 parsec in diameter, surrounding a smaller hot structure. As such a configuration of dust clouds would collapse in a time much shorter than the active phase of the AGN, this observation requires a continual input of kinetic energy to the cloud system from a source coexistent with the AGN.

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.

Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy

Aims. To test the dust torus model for active galactic nuclei directly, we study the extent and morphology of the nuclear dust distribution in the Circinus galaxy using high resolution interferometric observations in the mid-infrared. Methods. Observations were obtained with the MIDI instrument at the Very Large Telescope Interferometer. The 21 visibility points recorded are dispersed with a spectral resolution of λ/δλ ≈ 30 in the wavelength range from 8 to 13 µm. To interpret the data we used a stepwise approach of modelling with increasing complexity. The final model consists of two black body Gaussian distributions with dust extinction. Results. We find that the dust distribution in the nucleus of Circinus can be explained by two components, a dense and warm disk-like component of 0.4 pc size and a slightly cooler, geometrically thick torus component with a size of 2.0 pc. The disk component is oriented perpendicular to the ionisation cone and outflow and seems to show the silicate feature at 10 µm in emission. It coincides with a nuclear maser disk in orientation and size. From the energy needed to heat the dust, we infer a luminosity of the accretion disk of Lacc = 10 10 L� , which corresponds to 20% of the Eddington luminosity of the nuclear black hole. We find that the interferometric data are inconsistent with a simple, smooth and axisymmetric dust emission. The irregular behaviour of the visibilities and the shallow decrease of the dust temperature with radius provide strong evidence for a clumpy or filamentary dust structure. We see no evidence for dust reprocessing, as the silicate absorption profile is consistent with that of standard galactic dust. We argue that the collimation of the ionising radiation must originate in the geometrically thick torus component. Conclusions. Based on a great leap forward in the quality and quantity of interferometric data, our findings confirm the presence of a geometrically thick, torus-like dust distribution in the nucleus of Circinus, as required in unified schemes of Seyfert galaxies. Several aspects of our data require that this torus is irregular, or “clumpy”.

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.

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.

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.

Mid-infrared interferometry of the Mira variable RR Sco with the VLTI MIDI instrument

We present the results of the first mid-infrared interferometric observations of the Mira variable RR Sco with the MID-infrared Interferometer (MIDI) coupled to the European Southern Observatorys (ESO) Very Large Telescope Interferometer (VLTI), together with K-band observations using VLTI VINCI. The observations were carried out in June 2003, when the variability phase of the object was 0.6, using two unit telescopes (UT1 and UT3), as part of the Science Demonstration Time (SDT) program of the instrument. Projected baseline lengths ranged from 73 to 102 m, and a spectral resolution of 30 was employed in the observations, which enabled us to obtain the wavelength dependence of the visibility in the region be- tween 8 and 13 µm. The uniform-disk diameter was found to be 18 mas between 8 and 10 µm, while it gradually increases at wavelengths longer than 10 µm to reach 24 mas at 13 µm. The uniform-disk diameter between 8 and 13 µm is significantly larger than the K-band uniform-disk diameter of 10.2 ± 0.5 mas measured using VLTI VINCI with projected baseline lengths of 15-16 m, three weeks after the MIDI observations. Our model calculations show that optically thick emission from a warm molecular envelope consisting of H2O and SiO can cause the apparent mid-infrared diameter to be much larger than the con- tinuum diameter. We find that the warm molecular envelope model extending to ∼2.3 Rwith a temperature of ∼1400 K and column densities of H2 Oa nd SiO of 3× 10 21 cm −2 and 1 × 10 20 cm −2 , respectively, can reproduce the observed uniform-disk diameters between 8 and 10 µm. The observed increase of the uniform-disk diameter longward of 10 µm can be explained by an optically thin dust shell consisting of silicate and corundum grains. The inner radius of the optically thin dust shell is derived to be 7-8 Rwith a temperature of ∼700 K, and the optical depth at 10 µm is found to be ∼0.025.

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.