No abstract

Context. The nearby high-mass star binary system θ 1 Ori C is the brightest and most massive of the Trapezium OB stars at the core of the Orion Nebula Cluster, and it represents a perfect laboratory to determine the fundamental parameters of young hot stars and to constrain the distance of the Orion Trapezium Cluster. Aims. By tracing the orbital motion of the θ 1 Ori C components, we aim to refine the dynamical orbit of this important binary system. Methods. Between January 2007 and March 2008, we observed θ 1 Ori C with VLTI/AMBER near-infrared (H-and K-band) longbaseline interferometry, as well as with bispectrum speckle interferometry with the ESO 3.6 m and the BTA 6 m telescopes (Band V -band). Combining AMBER data taken with three different 3-telescope array configurations, we reconstructed the first VLTI/AMBER closure-phase aperture synthesis image, showing the θ 1 Ori C system with a resolution of ∼2 mas. To extract the astrometric data from our spectrally dispersed AMBER data, we employed a new algorithm, which fits the wavelength-differential visibility and closure phase modulations along the H-and K-band and is insensitive to calibration errors induced, for instance, by changing atmospheric conditions. Results. Our new astrometric measurements show that the companion has nearly completed one orbital revolution since its discovery in 1997. The derived orbital elements imply a short-period (P ≈ 11.3 yr) and high-eccentricity orbit (e ≈ 0.6) with periastron passage around 2002.6. The new orbit is consistent with recently published radial velocity measurements, from which we can also derive the first direct constraints on the mass ratio of the binary components. We employ various methods to derive the system mass (M system = 44 ± 7 M ) and the dynamical distance (d = 410 ± 20 pc), which is in remarkably good agreement with recently published trigonometric parallax measurements obtained with radio interferometry.

Context. Located in the Orion Trapezium cluster, θ 1 Ori C is one of the youngest and nearest high-mass stars (O5-O7) and known to be a close binary. Aims. By tracing its orbital motion, we aim to determine the orbit and dynamical mass of the system, yielding a characterization of the individual components and, ultimately, also new constraints for stellar evolution models in the high-mass regime. Methods. Using new multi-epoch visual and near-infrared bispectrum speckle interferometric observations obtained at the BTA 6 m telescope, and IOTA near-infrared long-baseline interferometry, we traced the orbital motion of the θ 1 Ori C components over the interval 1997.8 to 2005.9, covering a significant arc of the orbit. Besides fitting the relative position and the flux ratio, we applied aperture synthesis techniques to our IOTA data to reconstruct a model-independent image of the θ 1 Ori C binary system. Results. The orbital solutions suggest a highly eccentricity (e ≈ 0.91) and short-period (P ≈ 10.9 yrs) orbit. As the current astrometric data only allows rather weak constraints on the total dynamical mass, we present the two best-fit orbits. Of these two, the one implying a system mass of 48 M ⊙ and a distance of 434 pc to the Trapezium cluster can be favored. When also taking the measured flux ratio and the derived location in the HR-diagram into account, we find good agreement for all observables, assuming a spectral type of O5.5 for θ 1 Ori C1 (M = 34.0 M ⊙ , T eff = 39 900 K) and O9.5 for C2 (M = 15.5 M ⊙ , T eff = 31 900 K). Using IOTA, we also obtained first interferometric observations on θ 1 Ori D, finding some evidence for a resolved structure, maybe by a faint, close companion. Conclusions. We find indications that the companion C2 is massive itself, which makes it likely that its contribution to the intense UV radiation field of the Trapezium cluster is non-negligible. Furthermore, the high eccentricity of the preliminary orbit solution predicts a very small physical separation during periastron passage (∼ 1.5 AU, next passage around 2007.5), suggesting strong wind-wind interaction between the two O stars.

Abstract. We present bispectrum speckle interferometry of the massive protostellar object AFGL 2591 in the near-infrared K-band. Our reconstructed image of the outflow cavity of AFGL 2591 has a resolution of 170 mas, corresponding to physical scales of ∼170 AU at the distance of the object, and shows the loops which extend from the bright, compact source in unprecedented detail. The central source is clearly resolved and has an uniform-disk diameter of ∼40 mas (40 AU). We use 2D radiation transfer simulations to show that the resolved structure probably corresponds to the inner rim of a geometrically thick circumstellar disk or envelope at the dust sublimation radius. Our image also reveals a structure that might represent an edge-on circumstellar disk around one of the other young stellar objects near AFGL 2591.

Abstract. We present new near-infrared (JHK) bispectrum speckle-interferometry monitoring of the carbon star IRC+10216 obtained between 1999 and 2001 with the SAO 6 m telescope. The J-, H-, and K-band resolutions are 50 mas, 56 mas, and 73 mas, respectively. The total sequence of K-band observations covers now 8 epochs from 1995 to 2001 and shows the dynamic evolution of the inner dust shell. The present observations show that the appearance of the dust shell has considerably changed compared to the epochs of 1995 to 1998. Four main components within a 0. 2 radius can be identified in the K-band images. The apparent separation of the two initially brightest components A and B increased from ∼191 mas in 1995 to ∼351 mas in 2001. Simultaneously, component B has been fading and almost disappeared in 2000 whereas the initially faint components C and D became brighter (relative to peak intensity). The changes of the images can be related to changes of the optical depth caused, for instance, by mass-loss variations or new dust condensation in the wind. Our recent two-dimensional radiative transfer model of IRC +10216 suggests that the observed relative motion of components A and B is not consistent with the outflow of gas and dust at the well-known terminal wind velocity of 15 km s −1 . The apparent motion with a deprojected velocity of 19 km s −1 on average and of recently 27 km s −1 appears to be caused by a displacement of the dust density peak due to dust evaporation in the optically thicker and hotter environment. The present monitoring, covering more than 3 pulsation periods, shows that the structural variations are not related to the stellar pulsation cycle in a simple way. This is consistent with the predictions of hydrodynamical models that enhanced dust formation takes place on a timescale of several pulsation periods. The timescale of the fading of component B can well be explained by the formation of new dust in the circumstellar envelope.

Abstract. The scientific preparation of the Gaia mission encompasses both the data reduction algorithms and the generation of simulated data which have to be as realistic as possible. In this respect, binaries and multiple stars are a mandatory component in the simulation of the Milky Way as they impact on the performance tests of the on-ground data processing. The ingredients for the simulation of multiple stars are described and the predictions are compared to observed data or outcomes from stellar multiplicity studies.

We present speckle masking observations ([2], [4]) of Seyfert galaxies with the Russian 6 m telescope. Diffraction-limited resolution of 76 mas in the K-band was obtained for the first time. This resolution is similar to the resolution of recent MERLIN and VLA observations of galactic centers, thus allowing us to study the radio-IR spectrum of the same structures. Figure 1 shows the decreasing K-band visibility function of NGC 1068 and the contour plot of our reconstructed image ([5]). The results show that NGC 1068 is resolved with a FWHM diameter of 30 mas or 2 pc for an assumed Gaussian flux distribution. The image is elongated in northern direction, which is approximately the direction of the radio jet. In the right panel of figure 1 the observed flux values at 5, 15 and 22 GHz (from [3]) are plotted together with our K-band flux. The spectral index between 5 GHz and the K-band is approximately 1/3. This spectrum can be explained by synchrotron emission of quasi-monoenergetic relativistic electrons (as for our Galactic Center by [1]). Assuming that the observed flux is mainly nuclear light (from, for example, scattering lobes above and below a torus, without absorption and re-emission) we use the same synchrotron model as has been used for the Galactic Center to explain our data. With this model, we find a source radius of R ~ 10 15 cm, a magnetic field of Β ~ 11 G, a electron number density of n e ~ 1.110 3 cm -3 and a mean electron energy of rsj 2.7 GeV. The corresponding model spectrum is shown in the right panel of fig. 1. The observed flux value at 2.2 μια lies slightly above the model spectrum. This could be caused by flux contributions from additional components, for example, a central stellar cluster, an accretion disk or thermal radiation from a dusty torus. We have also observed the central regions of other galaxies. For example, for NGC 4151, we found a dominant 103 Y. Soße (ed.), The Central Regions of the Galaxy and Galaxies, 103-104.

Abstract. We present diffraction-limited bispectrum speckle interferometry observations of four well-known Herbig Ae/Be (HAeBe) stars, LkHα 198, Elias 1, HK Ori and V380 Ori. For two of these, LkHα 198 and Elias 1, we present the first unambiguous detection of close companions. The plane of the orbit of the new LkHα 198 companion appears to be significantly inclined to the plane of the circumprimary disk, as inferred from the orientation of the outflow. We show that the Elias 1 companion may be a convective star, and suggest that it could therefore be the true origin of the X-ray emission from this object. In the cases of HK Ori and V380 Ori, we present new measurements of the relative positions of already-known companions, indicating orbital motion. For HK Ori, photometric measurements of the brightness of the individual components in four bands allowed us to decompose the system spectral energy distribution (SED) into the two separate component SEDs. The primary exhibits a strong infrared excess which suggests the presence of circumstellar material, whereas the companion can be modelled as a naked photosphere. The infrared excess of HK Ori A was found to contribute around two thirds of the total emission from this component, suggesting that accretion power contributes significantly to the flux. Submillimetre constraints mean that the circumstellar disk cannot be particularly massive, whilst the near-infrared data indicates a high accretion rate. Either the disk lifetime is very short, or the disk must be seen in an outburst phase.