S. Kraus

Disk and wind interaction in the young stellar object MWC 297 spatially resolved with VLTI/AMBER

The young stellar object MWC 297 is an embedded B1.5Ve star exhibiting strong hydrogen emission lines and a strong near-infrared continuum excess. This object has been observed with the VLT interferometer equipped with the AMBER instrument during its first commissioning run. VLTI/AMBER is currently the only near infrared interferometer which can observe spectrally dispersed visibilities. MWC 297 has been spatially resolved in the continuum with a visibility of

Visual/infrared interferometry of orion trapezium stars : preliminary dynamical orbit and aperture synthesis imaging of the θ1 orionis C system

0.50^{+0.08}_{-0.10}

Tracing the young massive high-eccentricity binary system

as well as in the Brgamma emission line where the visibility decrease to a lower value of

\theta^{\mathsf 1}

0.33\pm0.06

Orionis C through periastron passage

. This change in the visibility with the wavelength can be interpreted by the presence of an optically thick disk responsible for the visibility in the continuum and of a stellar wind traced by the Brgamma emission line and whose apparent size is 40% larger. We validate this interpretation by building a model of the stellar environment that combines a geometrically thin, optically thick accretion disk model consisting of gas and dust, and a latitude-dependent stellar wind outflowing above the disk surface. The continuum emission and visibilities obtained from this model are fully consistent with the interferometric AMBER data. They agree also with existing optical, near-infrared spectra and other broad-band near-infrared interferometric visibilities. We also reproduce the shape of the visibilities in the Brgamma line as well as the profile of this line obtained at an higher spectral resolution with the VLT/ISAAC spectrograph, and those of the Halpha and Hbeta lines. The disk and wind models yield a consistent inclination of the system of approximately 20 degrees. A picture emerges in which MWC 297 is surrounded by an equatorial flat disk that is possibly still accreting and an outflowing wind which has a much higher velocity in the polar region than at the equator. The VLTI/AMBER unique capability to measure spectral visibilities therefore allows us for the first time to compare the apparent geometry of a wind with the disk structure in a young stellar system.

Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry Signatures of flow precession

Context. Located in the Orion Trapezium cluster, θ^1 Ori C is one of the youngest and nearest high-mass stars (O5-O7) known. Besides its unique properties as a magnetic rotator, the system is also 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. Furthermore, a dynamical parallax can be derived from the orbit, providing an independent estimate for the distance of the Trapezium cluster. 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.

VLTI/AMBER and VLTI/MIDI spectro-interferometric observations of the B(e) supergiant CPD¡57 - 2874 ? Size and geometry of the circumstellar envelope in the near- and mid-IR

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 -a ndK-band) longbaseline interferometry, as well as with bispectrum speckle interferometry with the ESO 3.6 m and the BTA 6 m telescopes (B � and 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 -a ndK-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 (Msystem = 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.

VLTI monitoring of the dust formation event of the Nova V1280 Sco

Context. NGC 7538 IRS1 is a high-mass (30 M⊙) protostar with a CO outflow, an associated ultracompact H II region, and a linear methanol maser structure, which might trace a Keplerian-rotating circumstellar disk. The directions of the various associated axes are misaligned with each other. Aims. We investigate the near-infrared morphology of the source to clarify the relations among the various axes. Methods. K’ -band bispectrum speckle interferometry was performed at two 6-meter-class telescopes—the BTA 6m telescope and the 6.5m MMT. Complementary IRAC images from theSpitzer Space Telescope Archive were used to relate the structures detected with the outflow at larger scales. Results. High-dynamic range images show fan-shaped outflow structur e in which we detect 18 stars and several blobs of diffuse emission. We interpret the misalignment of various outflow axes in the con text of a disk precession model, including numerical hydrodynamic simulations of the molecular emission. The precession period is∼ 280 years and its half-opening angle is∼ 40 ◦ . A possible triggering mechanism is noncoplanar tidal interaction of an (undiscovered) close companion with the circumbinary protostellar disk. Our observations resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with separation of 195 mas. We find indications for shock interaction bet ween the outflow activities in IRS1 and IRS2. Finally, we find prominent sites of star formation at the interface between two bubble-like structures in NGC 7538, suggestive of a triggered star formation scenario. Conclusions. Indications of outflow precession have been discovered to da te in a number of massive protostars, all with large precessi on angles (∼ 20‐45 ◦ ). This might explain the difference between the outflow widt hs in low- and high-mass stars and add support to a common collimation mechanism.

An asymmetry detected in the disk of Kappa CMa with the AMBER/VLTI

We present the first high spatial and spectral observations of the circumstellar envelope (CSE) of a B[e] supergiant (CPD

Resolving the asymmetric inner wind region of the yellow hypergiant IRC +10420 with VLTI/AMBER in low and high spectral resolution mode

-57 2874