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 L acc = 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".

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.

The asymptotic giant branch star R Sculptoris is surrounded by a detached shell of dust and gas 1,2 . The shell originates from a thermal pulse during which the star undergoes a brief period of increased mass loss 3,4 . It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse − parameters that determine the lifetime on the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3". What was hitherto thought to be only a thin, spherical shell with a clumpy structure, is revealed to contain a spiral structure. Spiral structures associated with circumstellar envelopes have been seen previously, from which it was concluded that the systems must be binaries 5,6,7,8 . Using the data, combined with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse ≈1800 years ago, lasting ≈200 years. About 3×10 !3 M ! of mass was ejected at a velocity of 14.3 km s −1 and at a rate ≈30 times higher than the prepulse mass-loss rate. This shows that ≈3 times more mass is returned to the interstellar medium during and immediately after a pulse than previously thought. The detached shell around R Sculptoris was observed in CO(J = 3 − 2) emission at 345 GHz using the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 0 operations (Fig.1, and supplementary information). The data clearly show the well-centered detached shell with a radius of 18.5", and reveal a spiral structure extending from the central star outwards to the shell. Previous observations of R Sculptoris show structure in the form of clumps. However, this was interpreted as clumpy material within the shell itself, and not as a structure interior to the shell 2 . Until now no clear signs of binary companions have been observed in the detached shell sources (with a possible exception for the detached shell around TT Cyg 9 ). The observed structure around R Sculptoris, however, indicates the presence of a companion, shaping the stellar wind into a spiral shell structure 8 . Smoothed particle hydrodynamics (SPH) models show that a wide binary companion can have a significant effect in the shaping of the wind, leading to elliptical and spiral structures (e.g. as observed in the case of the envelope of AFGL 3068) 5,6 . The observed shapes of the circumstellar envelopes (CSEs) around binary AGB stars depend on the physical parameters of the binary system (e.g., separation and mass ratio 10 ), the density contrasts imprinted on the wind, the temperatures in the CSE, the viewing angle, and, in the case of the gas, the chemistry and excitation 11 . The temporal variations of the mass-loss-rate and the expansion velocity further affect the structure of the CSE. Hence, the observed spiral structure and detached shell allow us to measure these important properties, and to directly link them to th...

Aims. We present the first multi-epoch study that includes concurrent mid-infrared and radio interferometry of an oxygen-rich Mira star. Results. The modeling of our MIDI data results in phase-dependent continuum photospheric angular diameters of 9.0 ± 0.3 mas (phase 0.42), 7.9 ± 0.1 mas (0.55), 9.7 ± 0.1 mas (1.16), and 9.5 ± 0.4 mas (1.27). The dust shell can best be modeled with Al 2 O 3 grains using phase-dependent inner boundary radii between 1.8 and 2.4 photospheric radii. The dust shell appears to be more compact with greater optical depth near visual minimum (τ V ∼ 2.5), and more extended with lower optical depth after visual maximum (τ V ∼ 1.5). The ratios of the 43.1 GHz/42.8 GHz SiO maser ring radii to the photospheric radii are 2.2 ± 0.3/2.1 ± 0.2 (phase 0.44), 2.4 ± 0.3/2.3 ± 0.4 (0.55), and 2.1 ± 0.3/1.9 ± 0.2 (1.15). The maser spots mark the region of the molecular atmospheric layers just beyond the steepest decrease in the mid-infrared model intensity profile. Their velocity structure indicates a radial gas expansion. Conclusions. S Ori shows significant phase-dependences of photospheric radii and dust shell parameters. Al 2 O 3 dust grains and SiO maser spots form at relatively small radii of ∼1.8−2.4 photospheric radii. Our results suggest increased mass loss and dust formation close to the surface near the minimum visual phase, when Al 2 O 3 dust grains are co-located with the molecular gas and the SiO maser shells, and a more expanded dust shell after visual maximum. Silicon does not appear to be bound in dust, as our data show no sign of silicate grains.

Context. Asymptotic giant branch (AGB) stars are one of the major sources of dust in the universe. The formation of molecules and dust grains and their subsequent expulsion into the interstellar medium via strong stellar winds is under intense investigation. This is in particular true for oxygen-rich stars, for which the path of dust formation has remained unclear. Aims. We conducted spatially and spectrally resolved mid-infrared multi-epoch interferometric observations to investigate the dust formation process in the extended atmospheres of oxygen-rich AGB stars. Methods. We observed the Mira variable AGB stars S Ori, GX Mon, and R Cnc between February 2006 and March 2009 with the MIDI instrument at the VLT interferometer. We compared the data to radiative transfer models of the dust shells, where the central stellar intensity profiles were described by dust-free dynamic model atmospheres. We used Al 2 O 3 and warm silicate grains, following earlier studies in the literature. Results. Our S Ori and R Cnc data could be well described by an Al 2 O 3 dust shell alone, and our GX Mon data by a mix of an Al 2 O 3 and a silicate shell. The best-fit parameters for S Ori and R Cnc included photospheric angular diameters Θ Phot of 9.7 ± 1.0 mas and 12.3 ± 1.0 mas, optical depths τ V (Al 2 O 3 ) of 1.5 ± 0.5 and 1.35 ± 0.2, and inner radii R in of 1.9 ± 0.3 R Phot and 2.2 ± 0.3 R Phot , respectively. Best-fit parameters for GX Mon were Θ Phot = 8.7 ± 1.3 mas, τ V (Al 2 O 3 ) = 1.9 ± 0.6, R in (Al 2 O 3 ) = 2.1 ± 0.3 R Phot , τ V (silicate) = 3.2 ± 0.5, and R in (silicate) = 4.6 ± 0.2 R Phot . Our data did not show evidence of intra-cycle and cycle-to-cycle variability or of asymmetries within the error-bars and within the limits of our baseline and phase coverage. Conclusions. Our model fits constrain the chemical composition and the inner boundary radii of the dust shells, as well as the photospheric angular diameters. Our interferometric results are consistent with Al 2 O 3 grains condensing close to the stellar surface at about 2 stellar radii, co-located with the extended atmosphere and SiO maser emission, and warm silicate grains at larger distances of about 4-5 stellar radii. We verified that the number densities of aluminum can match that of the best-fit Al 2 O 3 dust shell near the inner dust radius in sufficiently extended atmospheres, confirming that Al 2 O 3 grains can be seed particles for the further dust condensation. Together with literature data of the mass-loss rates, our sample is consistent with a hypothesis that stars with low mass-loss rates form primarily dust that preserves the spectral properties of Al 2 O 3 , and stars with higher mass-loss rate form dust with properties of warm silicates.