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Belonging to the group of B[e] stars, V921 Scorpii is associated with a strong infrared excess and permitted and forbidden line emission, indicating the presence of low- and high-density circumstellar gas and dust. Many aspects of V921 Sco and other B[e] stars still remain mysterious, including their evolutionary state and the physical conditions resulting in the class-defining characteristics. In this paper, we employ VLTI/AMBER spectro-interferometry in order to reconstruct high-resolution (lambda/2B=0.0013") model-independent interferometric images for three wavelength bands around 1.65, 2.0, and 2.3 micrometer. In our images, we discover a close (25.0+/-0.8 milliarcsecond, corresponding to 29+/-0.9 AU at 1.15 kpc) companion around V921 Sco. Between two epochs in 2008 and 2009, we measure orbital motion of 7 degrees, implying an orbital period of about 35 years (for a circular orbit). Around the primary star, we detect a disk-like structure with indications for a radial temperature gradient. The polar axis of this AU-scale disk is aligned with the arcminute-scale bipolar nebula in which V921 Sco is embedded. Using Magellan/IMACS imaging, we detect multi-layered arc-shaped sub-structure in the nebula, suggesting episodic outflow activity from the system with a period of about 25 years, roughly matching the estimated orbital period of the companion. Our study supports the hypothesis that the B[e] phenomenon is related to dynamical interaction in a close binary system., Comment: 7 pages, 2 figures, accepted by The Astrophysical Journal Letters

Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared-excess and emission lines. The phenomena involved in the disk formation still remain highly debated. Aims. To progress in the understanding of the physical process or processes responsible for the mass ejections and test the hypothesis that they depend on the stellar parameters, we initiated a survey on the circumstellar environment of the brightest Be stars. Methods. To achieve this goal, we used spectro-interferometry, the only technique that combines high spectral (R=12000) and high spatial ($\theta_{\rm min}$=4\,mas) resolutions. Observations were carried out at the Paranal observatory with the VLTI/AMBER instrument. We concentrated our observations on the Br$\gamma$ emission line to be able to study the kinematics within the circumstellar disk. Our sample is composed of eight bright classical Be stars : $\alpha$ Col, $\kappa$ CMa, $\omega$ Car, p Car, $\delta$ Cen, $\mu$ Cen, $\alpha$ Ara, and \textit{o} Aqr. Results. We managed to determine the disk extension in the line and the nearby continuum for most targets. We also constrained the disk kinematics, showing that it is dominated by rotation with a rotation law close to the Keplerian one. Our survey also suggests that these stars are rotating at a mean velocity of V/V$_{\rm c}$\,=\,0.82\,$\pm$\,0.08. This corresponds to a rotational rate of $\Omega/\Omega_{\rm c}$\,=\,0.95\,$\pm$\,0.02 Conclusions. We did not detect any correlation between the stellar parameters and the structure of the circumstellar environment. Moreover, it seems that a simple model of a geometrically thin Keplerian disk can explain most of our spectrally resolved K-band data. Nevertheless, some small departures from this model have been detected for at least two objects (i.e, $\kappa$ CMa and $\alpha$ Col). Finally, our Be stars sample suggests that rotation is the main physical process driving the mass-ejection. Nevertheless, smaller effects from other mechanisms have to be taken into account to fully explain how the residual gravity is compensated., Comment: Astronomy and Astrophysics (2011) Accepted

We contribute some information towards finding a general algorithm for constructing, for a given profinite group, $G$, a compact connected space, $X$, such that the full homeomorphism group, $H(X)$, with the compact-open topology is isomorphic to $G$ as a topological group. It is proposed that one should find a compact topological oriented graph $\Gamma$ such that $G\cong Aut(\Gamma)$. The replacement of the edges of $\Gamma$ by rigid continua should work as is exemplified in various instances where discrete graphs were used. It is shown here that the strategy can be implemented for profinite monothetic groups $G$.

Due to the recent dramatic technological advances, infrared interferometry can now be applied to new classes of objects, resulting in exciting new science prospects, for instance, in the area of high-mass star formation. Although extensively studied at various wavelengths, the process through which massive stars form is still only poorly understood. For instance, it has been proposed that massive stars might form like low-mass stars by mass accretion through a circumstellar disk/envelope, or otherwise by coalescence in a dense stellar cluster. After discussing the technological challenges which result from the special properties of these objects, we present first near-infrared interferometric observations, which we obtained on the massive YSO IRAS 13481-6124 using VLTI/AMBER infrared long-baseline interferometry and NTT speckle interferometry. From our extensive data set, we reconstruct a model-independent aperture synthesis image which shows an elongated structure with a size of 13x19 AU, consistent with a disk seen under an inclination of 45 degree. The measured wavelength-dependent visibilities and closure phases allow us to derive the radial disk temperature gradient and to detect a dust-free region inside of 9.5 AU from the star, revealing qualitative and quantitative similarities with the disks observed in low-mass star formation. In complementary mid-infrared Spitzer and sub-millimeter APEX imaging observations we detect two bow shocks and a molecular out ow which are oriented perpendicular to the disk plane and indicate the presence of a bipolar outflow emanating from the inner regions of the system., Comment: 11 pages, 4 figures, to appear in Proceedings of SPIE Astronomical Telescopes and Instrumentation 2010

Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, nonspherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical unit-scale distribution of hot material around a high-mass (approx. 20 solar masses) young stellar object. The image shows an elongated structure with a size of about 13 x 19 astronomical units, consistent with a disk seen at an inclination angle of 45 degree. Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system., Comment: 13 pages, 2 figures, published in July 15 issue of Nature; Supplementary Information document available at http://www.nature.com/nature/journal/v466/n7304/extref/nature09174-s1.pdf

We report near-IR spectroscopic observations of the Eta Carinae massive binary system during 2008-2009 using VLT/CRIRES. We detect a strong, broad absorption wing in He I 10833 extending up to -1900 km/s across the 2009.0 spectroscopic event. Archival HST/STIS ultraviolet and optical data shows a similar high-velocity absorption (up to -2100 km/s) in the UV resonance lines of Si IV 1394, 1403 across the 2003.5 event. UV lines from low-ionization species, such as Si II 1527, 1533 and C II 1334, 1335, show absorption up to -1200 km/s, indicating that the absorption with v from -1200 to -2100 km/s originates in a region markedly faster and more ionized than the nominal wind of the primary star. Observations obtained at the OPD/LNA during the last 4 spectroscopic cycles (1989-2009) also display high-velocity absorption in He I 10833 during periastron. Based on the OPD/LNA dataset, we determine that material with v < -900 km/s is present in the phase range 0.976 < phi < 1.023 of the spectroscopic cycle, but absent in spectra taken at phi < 0.947 and phi > 1.049. Therefore, we constrain the duration of the high-velocity absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We suggest that the high-velocity absorption originates from shocked gas in the wind-wind collision zone, at distances of 15 to 45 AU in the line-of-sight to the primary star. Using 3-D hydrodynamical simulations of the wind-wind collision zone, we find that the dense high-velocity gas is in the line-of-sight to the primary star only if the binary system is oriented in the sky so that the companion is behind the primary star during periastron, corresponding to a longitude of periastron of omega ~ 240 to 270 degrees. We study a possible tilt of the orbital plane relative to the Homunculus equatorial plane and conclude that our data are broadly consistent with orbital inclinations in the range i=40 to 60 degrees., Comment: 18 pages, 15 figures, accepted for publication in A&A; high-resolution PDF version available also at http://www.mpifr.de/staff/jgroh/etacar.html

We show that a compact group $G$ has finite conjugacy classes, i.e., is an FC-group if and only if its center $Z(G)$ is open if and only if its commutator subgroup $G'$ is finite. Let $d(G)$ denote the Haar measure of the set of all pairs $(x,y)$ in $G \times G$ for which $[x,y] = 1$; this, formally, is the probability that two randomly picked elements commute. We prove that $d(G)$ is always rational and that it is positive if and only if $G$ is an extension of an FC-group by a finite group. This entails that $G$ is abelian by finite. The proofs involve measure theory, transformation groups, Lie theory of arbitrary compact groups, and representation theory of compact groups. Examples and references to the history of the discussion are given at the end of the paper., Comment: 17 pages; we have cut some points ; to appear in Math. Proc. Cambridge Phil. Soc

We report the first resolved bispectrum speckle interferometry of the symbiotic binary CH Cyg. The measured component separation, $\rho=42 \pm 2$ mas, is consistent with the one derived from the known spectroscopic orbit and distance. In particular, our result implies a total mass of the binary of $M_{\rm t}=M_{\rm g}+M_{\rm wd}=3.7^{+3.5}_{-1.7} \rm M_{\sun}$, which is in good agreement with the value $M_{\rm t}=2.7^{+1.2}_{-0.6} \rm M_{\sun}$ derived from the spectroscopic orbit solution for the red giant and evolutionary contraints. We also show that the radio jets and the bipolar outflow are not orthogonal to the orbital plane of the binary system., Comment: 5 pages, 4 figures, submitted to MNRAS

Most Wolf-Rayet stars (WR) of WC9 sub-type exhibit a dusty circumstellar envelope, but it is still a matter of debate how dust can form in their harsh environment. In a few cases, a pinwheel-like structure of the dusty envelope has been detected. Therefore, it has been suggested that dust formation in all dusty WR stars might be linked to colliding winds in a binary system. We probed the innermost region of the circumstellar dust shell of the deeply embedded WR star WR 118. We carried out spectro-interferometric observations using the AMBER instrument of ESO's Very Large Telescope Interferometer in low-spectral resolution mode (R = 35). The K-band observations were obtained with three 1.8 m telescopes spanning projected baselines between 9.2 and 40.1 m. At high spatial frequencies, the AMBER visibilities exhibit a prominent lobe, indicating that the envelope contains one or several zones with a large local intensity gradient. The strong closure phase signal clearly shows that the circumstellar envelope of WR 118 can only be described by an asymmetric intensity distribution. We show that a pinwheel nebula seen at low inclination is consistent with the AMBER data. Its size was determined to be 13.9+-1.1 mas. WR 118 possibly harbors a pinwheel nebula, which suggests a binary nature of the system. According to our best model, the period of the system would be ~60 days (for d=3 kpc), making WR 118 the shortest-period pinwheel nebula known so far., Comment: Letter accepted in A&A for publication

Massive stars deeply influence their surroundings by their luminosity and the injection of kinetic energy. So far, they have mostly been studied with spatially unresolved observations, although evidence of geometrical complexity of their wind are numerous. Interferometry can provide spatially resolved observations of massive stars and their immediate vicinity. Specific geometries (disks, jets, latitude-dependent winds) can be probed by this technique. The first observation of a Wolf-Rayet (WR) star (\gamma^2 Vel) with the AMBER/VLTI instrument yielded to a re-evaluation of its distance and an improved characterization of the stellar components, from a very limited data-set. This motivated our team to increase the number of WR targets observed with AMBER. We present here new preliminary results that encompass several spectral types, ranging from early WN to evolved dusty WC. We present unpublished data on WR79a, a massive star probably at the boundary between the O and Wolf- Rayet type, evidencing some Wolf-Rayet broad emission lines from an optically thin wind. We also present new data obtained on \gamma^2 Vel that can be compared to the up-to-date interferometry-based orbital parameters from North et al. (2007). We discuss the presence of a wind-wind collision zone in the system and provide preliminary analysis suggesting the presence of such a structure in the data. Then, we present data obtained on 2 dusty Wolf-Rayet stars: WR48a-b and WR118, the latter exhibiting some clues of a pinwheel-like structure from the visibility variations., Comment: This paper will be published in the proceeding of SPIE ``astronomical Telescopes and Instrumentation: Optical and Infrared Interferometry''

We present N-band spectro-interferometric observations of the red supergiant WOH G64 in the Large Magellanic Cloud (LMC) using MIDI at the Very Large Telescope Interferometer (VLTI). The location of WOH G64 on the H-R diagram based on the previously estimated luminosities is in serious disagreement with the current stellar evolution theory. The dust envelope around WOH G64 has been spatially resolved with a baseline of ~60 m--the first MIDI observations to resolve an individual stellar source in an extragalactic system. The observed N-band visibilities show a slight decrease from 8 to 10 micron and a gradual increase longward of 10 micron, reflecting the 10 micron silicate feature in self-absorption. The visibilities measured at four position angles differing by ~60 degrees but at approximately the same baseline length (~60 m) do not show a noticeable difference, suggesting that the object appears nearly centrosymmetric. The observed N-band visibilities and spectral energy distribution can be reproduced by an optically and geometrically thick silicate torus model viewed close to pole-on. The luminosity of the central star is derived to be 2.8 x 10^5 Lsun, which is by a factor of 2 lower than the previous estimates based on spherical models. The lower luminosity newly derived from our MIDI observations and two-dimensional modeling brings the location of WOH G64 on the H-R diagram in much better agreement with theoretical evolutionary tracks for a 25 Msun star. We also identify the H2O absorption features at 2.7 and 6 micron in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope. The 2.7 micron feature originates in the photosphere and/or the extended molecular layers, while the 6 micron feature is likely to be of circumstellar origin., Comment: 9 pages, 6 figures, accepted for publication in A&A

It is shown that a closed solvable subgroup of a connected Lie group is compactly generated. In particular, every discrete solvable subgroup of a connected Lie group is finitely generated. Generalizations to locally compact groups are discussed as far as they carry.

AMBER is the General User near-infrared focal instrument of the Very Large Telescope interferometer. Its specifications are based on three key programs on Young Stellar Objects, Active Galactic Nuclei central regions, masses and spectra of hot Extra Solar Planets. It has an imaging capacity because it combines up to three beams and very high accuracy measurement are expected from the spatial filtering of beams by single mode fibers and the comparison of measurements made simultaneously in different spectral channels., Comment: 10 pages

We present two-dimensional (2D) radiative transfer modeling of IRC+10216 at selected moments of its evolution in 1995-2001, which correspond to three epochs of our series of 8 near-infrared speckle images (Osterbart et al. 2000, Weigelt et al. 2002). The high-resolution images obtained over the last 5.4 years revealed the dynamic evolution of the subarcsecond dusty environment of IRC+10216 and our recent time-independent 2D radiative transfer modeling reconstructed its physical properties at the single epoch of January 1997 (Men'shchikov et al. 2001). Having documented the complex changes in the innermost bipolar shell of the carbon star, we incorporate the evolutionary constraints into our new modeling to understand the physical reasons for the observed changes. The new calculations imply that during the last 50 years, we have been witnessing an episode of a steadily increasing mass loss from the central star, from Mdot ~ 10^-5 Msun/yr to the rate of Mdot ~ 3x10^-4 Msun/yr in 2001. The rapid increase of the mass loss of IRC+10216 and continuing time-dependent dust formation and destruction caused the observed displacement of the initially faint components C and D and of the bright cavity A from the star which has almost disappeared in our images in 2001. Increasing dust optical depths are causing strong backwarming that leads to higher temperatures in the dust formation zone, displacing the latter outward with a velocity v_T ~ 27 km/s due to the evaporation of the recently formed dust grains. This shift of the dust density peak in the bipolar shell mimics a rapid radial expansion, whereas the actual outflow has probably a lower speed v < v_inf ~ 15 km/s. The model predicts that the star will remain obscured until Mdot starts to drop back to lower values in the dust formation zone., Comment: 10 pages, 6 figures, accepted by Astronomy and Astrophysics, also available at http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.html

With our new speckle imaging polarimeter we have obtained first polarimetric images with sub-arcsecond resolution of the Luminous Blue Variable eta Carinae in the Halpha line. The polarization patterns at the 3'' scale match well earlier conventional imaging photometry and can be interpreted as Mie scattering. In centered long-exposure images we detected in polarized light a bar in the NE part of the equatorial plane of eta Carinae. High-resolution 0.11'' polarimetric speckle reconstructions reveal a compact structure elongated in the same direction which is consistent, in degree and position angle of the polarisation, with the presence of a circumstellar, equatorial disk. The degree of polarization of the previously discovered speckle objects and the Halpha arm is relatively low (~10%) and thus may indicate a position within the equatorial plane. We also discovered a highly polarized (20%-40%) bipolar structure along the major axis of the Homunculus nebula which can be traced down to the sub-arcsecond scale. This is probably the inner part of a bipolar outflow into the Homunculus., Comment: Astron. & Astrophys. Letters, in press, LaTex (laa.sty), 4 pages, 5 PS-figures, also available at http://www.astro.umd.edu/~hfalcke/publications.html or by request

We answer a question raised by V.G.Pestov in the affirmative., Comment: 2 pages