S. Morel

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.

PIONIER: a 4-telescope visitor instrument at VLTI

PIONIER stands for Precision Integrated-Optics Near-infrared Imaging ExpeRiment. It combines four 1.8m Auxilliary Telescopes or four 8m Unit Telescopes of the Very Large Telescope Interferometer (ESO, Chile) using an integrated optics combiner. The instrument has been integrated at IPAG starting in December 2009 and commissioned at the Paranal Observatory in October 2010. It provides scientific observations since November 2010. In this paper, we detail the instrumental concept, we describe the standard operational modes and the data reduction strategy. We present the typical performance and discuss how to improve them. This paper is based on laboratory data obtained during the integrations at IPAG, as well as on-sky data gathered during the commissioning at VLTI. We illustrate the imaging capability of PIONIER on the binaries deltaSco and HIP11231. PIONIER provides 6 visibilities and 3 independent closure phases in the H band, either in a broadband mode or with a low spectral dispersion (R=40), using natural light (i.e. unpolarized). The limiting magnitude is Hmag=7 in dispersed mode under median atmospheric conditions (seeing 3ms) with the 1.8m Auxiliary Telescopes. We demonstrate a precision of 0.5deg on the closure phases. The precision on the calibrated visibilities ranges from 3 to 15% depending on the atmospheric conditions. PIONIER has been installed and successfully tested as a visitor instrument for the VLTI. It permits high angular resolution imaging studies at an unprecedented level of sensitivity. The successful combination of the four 8m Unit Telescopes in March 2011 demonstrates that VLTI is ready for 4-telescope operation.

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.

VLBA observations of SiO masers towards Mira variable stars

We present new total intensity and linear polarization VLBA observations of the ν = 2a ndν = 1 J = 1−0 maser transitions of SiO at 42.8 and 43.1 GHz in a number of Mira variable stars over a substantial fraction of their pulsation periods. These observations were part of an observing program that also includes interferometric measurements at 2.2 and 3.6 micron (Mennesson et al. 2002); comparison of the results from different wavelengths allows studying the envelope independently of the poorly known distances to these stars. Nine stars were observed at from one to four epochs during 2001. The SiO emission is largely confined to rings which are smaller than the inner radius of the dust shells reported by Danchi et al. (1994). Two stars (U Orionis, R Aquarii) have maser rings with diameters corresponding to the size of the hot molecular layer as measured at 3.6 micron; in the other cases, the SiO rings are substantially larger. Variations of ring diameter for most, but not all stars, had an rms amplitude in agreement with the models of Humphreys et al. (2002) although the expected relationship between the diameter and pulsation phase was not seen. The ring diameter in U Orionis shows remarkably small variation. A correlation between the 2.2/3.6 µm diameter ratio with that of the SiO/3.6 µm diameter ratio is likely due to differences in the opacities at 2.2 and 3.6 µm in a molecular layer. A further correlation with the inner size of the dust shell reported by Danchi et al. (1994) suggest some differences in the temperature structure. Clear evidence is seen in R Aquarii for an equatorial disk similar to that reported by Hollis et al. (2001); rotation is possibly also detected in S Coronae Boralis.

EVIDENCE FOR VERY EXTENDED GASEOUS LAYERS AROUND O-RICH MIRA VARIABLES AND M GIANTS

Nine bright O-rich Mira stars and five semiregular variable cool M giants have been observed with the Infrared and Optical Telescope Array (IOTA) interferometer in both K 0 (� 2.15 lm) and L 0 (� 3.8 lm) broadband filters, in most cases at very close variability phases. All of the sample Mira stars and four of the semiregular M giants show strong increases, from ’20% to ’100%, in measured uniform-disk (UD) diameters between the K 0 and L 0 bands. (A selection of hotter M stars does not show such a large increase.) There is no evidence that K 0 and L 0 broadband visibility measurements should be dominated by strong molecular bands, and cool expanding dust shells already detected around some of these objects are also found to be poor candidates for producing these large apparent diameter increases. Therefore, we propose that this must be a continuum or pseudocontinuum opacity effect. Such an apparent enlargement can be reproduced using a simple two-component model consisting of a warm (1500–2000 K), extended (up to ’3 stellar radii), optically thin (� ’ 0:5) layer located above the classical photosphere. The Planck weighting of the continuum emission from the two layers will suffice to make the L 0 UD diameter appear larger than the K 0 UD diameter. This twolayer scenario could also explain the observed variation of Mira UD diameters versus infrared wavelength— outside of strong absorption bands—as already measured inside the H, K, L, and N atmospheric windows. This interpretation is consistent with the extended molecular gas layers (H2O, CO, etc.) inferred around some of these objects from previous IOTA K 0 -band interferometric observations obtained with the Fiber Linked Unit for Optical Recombination (FLUOR) and from Infrared Space Observatory and high-resolution ground-based FTS infrared spectra. The two-component model has immediate implications. For example, the Mira photosphere diameters are smaller than previously recognized—this certainly implies higher effective temperatures, and it may favor fundamental mode pulsation. Also, the UD model fails generally to represent the brightness distribution and has very limited applicability for Mira stars. The presence of a very extended gas layer extending up to ’3 stellar radii seems now well established on a fair sample of asymptotic giant branch stars ranging from late-type giants to long-period variables, with some probable impact on stellar model atmospheres and mass-loss mechanisms. Subject headings: circumstellar matter — instrumentation: interferometers — stars: atmospheres — stars: variables: other — techniques: interferometric

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.

First Results with the IOTA3 Imaging Interferometer: The Spectroscopic Binaries λ Virginis and WR 140

We report the first spatially resolved observations of the spectroscopic binaries λ Vir and WR 140, including the debut of aperture-synthesis imaging with the upgraded three-telescope IOTA interferometer. Using IONIC-3, a new integrated optics beam combiner capable of a precise closure phase measurement, short observations were sufficient to extract the angular separation and orientation of each binary system and the component brightness ratio. Most notably, the underlying binary in the prototypical colliding-wind source WR 140 (WC7 + O4/O5) was found to have a separation of ~13 mas with a position angle of 152°, consistent with previous interpretations of the 2001 dust shell ejection only if the Wolf-Rayet star is fainter than the O star at 1.65 μm. We also highlight λ Vir, whose peculiar stellar properties of the Am star components will permit direct testing of current theories of tidal evolution when the full orbit is determined.

High angular resolution n-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument : Dusty environment spatially resolved

We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ∼36 mas (72 R� ) between 8 and 10 µm, while it steeply increases longward of 10 µm to reach ∼53 mas (106 R� )a t 13µm. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly – though not entirely satisfactorily – reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.