Aims. We present one-dimensional aperture synthesis imaging of the red supergiant Betelgeuse (α Ori) with VLTI/AMBER. We reconstructed for the first time one-dimensional images in the individual CO ...first overtone lines. Our aim is to probe the dynamics of the inhomogeneous atmosphere and its time variation. Methods. Betelgeuse was observed between 2.28 and 2.31 μm with VLTI/AMBER using the 16-32-48 m telescope configuration with a spectral resolution up to 12 000 and an angular resolution of 9.8 mas. The good nearly one-dimensional uv coverage allows us to reconstruct one-dimensional projection images (i.e., one-dimensional projections of the object’s two-dimensional intensity distributions). Results. The reconstructed one-dimensional projection images reveal that the star appears differently in the blue wing, line center, and red wing of the individual CO lines. The one-dimensional projection images in the blue wing and line center show a pronounced, asymmetrically extended component up to ~1.3 R⋆, while those in the red wing do not show such a component. The observed one-dimensional projection images in the lines can be reasonably explained by a model in which the CO gas within a region more than half as large as the stellar size is moving slightly outward with 0–5 km s-1, while the gas in the remaining region is infalling fast with 20–30 km s-1. A comparison between the CO line AMBER data taken in 2008 and 2009 shows a significant time variation in the dynamics of the CO line-forming region in the photosphere and the outer atmosphere. In contrast to the line data, the reconstructed one-dimensional projection images in the continuum show only a slight deviation from a uniform disk or limb-darkened disk. We derive a uniform-disk diameter of 42.05 ± 0.05 mas and a power-law-type limb-darkened disk diameter of 42.49 ± 0.06 mas and a limb-darkening parameter of (9.7 ± 0.5) × 10-2. This latter angular diameter leads to an effective temperature of 3690 ± 54 K for the continuum-forming layer. These diameters confirm that the near-IR size of Betelgeuse was nearly constant over the last 18 years, in marked contrast to the recently reported noticeable decrease in the mid-IR size. The continuum data taken in 2008 and 2009 reveal no or only marginal time variations, much smaller than the maximum variation predicted by the current three-dimensional convection simulations. Conclusions. Our two-epoch AMBER observations show that the outer atmosphere extending to ~1.3–1.4 R⋆ is asymmetric and its dynamics is dominated by vigorous, inhomogeneous large-scale motions, whose overall nature changes drastically within one year. This is likely linked to the wind-driving mechanism in red supergiants.
Aims. We present the first multi-epoch study that includes concurrent mid-infrared and radio interferometry of an oxygen-rich Mira star. Methods. We obtained mid-infrared interferometry of S Ori with ...VLTI/MIDI at four epochs in December 2004, February/March 2005, November 2005, and December 2005. We concurrently observed v =1, J =1-0 (43.1 GHz), and v =2, J =1-0 (42.8 GHz) SiO maser emission toward S Ori with the VLBA in January, February, and November 2005. The MIDI data are analyzed using self-excited dynamic model atmospheres including molecular layers, complemented by a radiative transfer model of the circumstellar dust shell. The VLBA data are reduced to the spatial structure and kinematics of the maser spots. Results. The modeling of our MIDI data results in phase- dependent continuum photospheric angular diameters of 9.0 \pm 0.3 mas (phase 0.42), 7.9 \pm 0.1 mas (0.55), 9.7 \pm 0.1 mas (1.16), and 9.5 \pm 0.4 mas (1.27). The dust shell can best be modeled with Al sub(2) O sub(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 ( \tau_V\sim 2.5), and more extended with lower optical depth after visual maximum ( \tau_V\sim 1.5). The ratios of the 43.1 GHz/42.8 GHz SiO maser ring radii to the photospheric radii are 2.2 \pm 0.3/2.1 \pm 0.2 (phase 0.44), 2.4 \pm 0.3/2.3 \pm 0.4 (0.55), and 2.1 \pm 0.3/1.9 \pm 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 sub(2) O sub(3) dust grains and SiO maser spots form at relatively small radii of similar to 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 sub(2) O sub(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.
Aims. We present spatially resolved, high-spectral resolution K-band observations of the red supergiant Betelgeuse (α Ori) using AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to ...probe inhomogeneous structures in the dynamical atmosphere of Betelgeuse. Methods. Betelgeuse was observed in the wavelength range between 2.28 and 2.31 μm with VLTI/AMBER using baselines of 16, 32, and 48 m. The spectral resolutions of 4800-12 000 allow us to study inhomogeneities seen in the individual CO first overtone lines. Results. Spectrally dispersed interferograms have been successfully obtained in the second, third, and fifth lobes, which represents the highest spatial resolution (9 mas) achieved for Betelgeuse. This corresponds to five resolution elements over its stellar disk. The AMBER visibilities and closure phases in the K-band continuum can be reasonably fitted by a uniform disk with a diameter of 43.19 ± 0.03 mas or a limb-darkening disk with 43.56 ± 0.06 mas and a limb-darkening parameter of (1.2 ± 0.07) $\times$ 10-1. These AMBER data and the previous K-band interferometric data taken at various epochs suggest that Betelgeuse seen in the K-band continuum shows much smaller deviations from the above uniform disk or limb-darkened disk than predicted by recent 3-D convection simulations for red supergiants. On the other hand, our AMBER data in the CO lines reveal salient inhomogeneous structures. The visibilities and phases (closure phases, as well as differential phases representing asymmetry in lines with respect to the continuum) measured within the CO lines show that the blue and red wings originate in spatially distinct regions over the stellar disk, indicating an inhomogeneous velocity field that makes the star appear different in the blue and red wings. Our AMBER data in the CO lines can be roughly explained by a simple model, in which a patch of CO gas is moving outward or inward with velocities of 10-15 km s-1, while the CO gas in the remaining region in the atmosphere is moving in the opposite direction at the same velocities. Also, the AMBER data are consistent with the presence of warm molecular layers (so-called MOLsphere) extending to ~1.4-1.5 $R_{\star}$ with a CO column density of ~1 $\times$ 1020 cm-2. Conclusions. Our AMBER observations of Betelgeuse are the first spatially resolved study of the so-called macroturbulence in a stellar atmosphere (photosphere and possibly MOLsphere as well) other than the Sun. The spatially resolved CO gas motion is likely to be related to convective motion in the upper atmosphere or intermittent mass ejections in clumps or arcs.
Aims. We aim to explore the photosphere of the very cool late-type star VX Sgr and in particular the characterization of molecular layers above the continuum forming photosphere. Methods. We obtained ...interferometric observations with the VLTI/AMBER interferometer using the fringe tracker FINITO in the spectral domain 1.45-2.50 m with a spectral resolution of and baselines ranging from 15 to 88 m. We performed independent image reconstruction for different wavelength bins and fit the interferometric data with a geometrical toy model. We also compared the data to 1D dynamical models of Miras atmosphere and to 3D hydrodynamical simulations of red supergiant (RSG) and asymptotic giant branch (AGB) stars. Results. Reconstructed images and visibilities show a strong wavelength dependence. The H-band images display two bright spots whose positions are confirmed by the geometrical toy model. The inhomogeneities are qualitatively predicted by 3D simulations. At m and in the region 2.35-2.50 m, the photosphere appears extended and the radius is larger than in the H band. In this spectral region, the geometrical toy model locates a third bright spot outside the photosphere that can be a feature of the molecular layers. The wavelength dependence of the visibility can be qualitatively explained by 1D dynamical models of Mira atmospheres. The best-fitting photospheric models show a good match with the observed visibilities and give a photospheric diameter of 0.50 mas. The H sub(2)O molecule seems to be the dominant absorber in the molecular layers. Conclusions. We show that the atmosphere of VX Sgr seems to resemble Mira/AGB star model atmospheres more closely than do RSG model atmospheres. In particular, we see molecular (water) layers that are typical of Mira stars.
Aims. 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). While the ...very high luminosity (∼$ 5 \times 10^5$ $L_{\odot}$) previously estimated for WOH G64 suggests that it is a very massive star with an initial mass of ~40 $M_{\odot}$, its low effective temperature (~3200 K) is in serious disagreement with the current stellar evolution theory. Methods. WOH G64 was observed with VLTI/MIDI using the UT2-UT3 and UT3-UT4 baseline configurations. Results. 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 μm and a gradual increase longward of ~10 μm, reflecting the 10 μm silicate feature in self-absorption. This translates into a steep increase of the uniform-disk diameter from 8 to 10 μm (from 18 to 26 mas) and a roughly constant diameter above 10 μm. The visibilities measured at four position angles differing by ~60° 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 \times 10^5$ $L_{\odot}$, which is by a factor of 2 lower than the previous estimates based on spherical models. We also identify the H2O absorption features at 2.7 and 6 μm in the spectra obtained with the Infrared Space Observatory and the Spitzer Space Telescope. The 2.7 μm feature originates in the photosphere and/or the extended molecular layers, while the 6 μm feature is likely to be of circumstellar origin. Conclusions. 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 $M_{\odot}$ star. However, the effective temperature is still somewhat too cool for the theory. The low effective temperature of WOH G64 places it very close to or even beyond the Hayashi limit, which implies that this object may be experiencing unstable, violent mass loss.
The main objective of the
AtmoFlow
experiment is the investigation of convective flows in the spherical gap geometry. Gaining fundamental knowledge on the origin and behavior of flow phenomena such ...as global cells and planetary waves is interesting not only from a meteorological perspective. Understanding the interaction between atmospheric circulation and a planet’s climate, be it Earth, Mars, Jupiter, or a distant exoplanet, contributes to various fields of research such as astrophysics, geophysics, fluid physics, and climatology. AtmoFlow aims to observe flows in a thin spherical gap that are subjected to a central force-field. The Earth’s own gravitational field interferes with a simulated central force-field with the given parameters of the model which makes microgravity conditions of
g
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1
0
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(e.g. on the ISS) necessary. Without losing its overall view on the complex physics, circulation in planetary atmospheres can be reduced to a simple model of a central gravitational field, the incoming and outgoing energy (e.g. radiation) and rotational effects. This strongly simplified assumption makes it possible to break some generic cases down to test models which can be investigated by laboratory experiments and numerical simulations. Varying rotational rates and temperature boundary conditions represent different types of planets. This is a very basic approach, but various open questions regarding local pattern formation or global planetary cells can be investigated with that setup. A concept has been defined for developing a payload that could be installed and utilized on-board the International Space Station (ISS). This concept is based on the microgravity experiment
GeoFlow
, which has been conducted successfully between 2008 and 2016 on the ISS. This paper addresses the scientific goals, the experimental setup, the concept for implementation of the AtmoFlow experiment on the ISS and first numerical results.
Aims.We present the first multi-epoch N-band spectro-interferometric observations of the carbon-rich Mira variable V Oph using MIDI at the ESO's Very Large Telescope Interferometer. Our aim is to ...study temporal variations of physical properties of the outer atmosphere and the circumstellar dust shell based on spectrally-dispersed N-band visibilities over the C2H2 (+HCN) features and the dust emission. Methods.Our MIDI observations were carried out at three different phases 0.18, 0.49, and 0.65, with three different baselines (projected baseline lengths of 42–124 m) using four 8.2 m Unit Telescopes (UT2-UT4, UT1-UT4, and UT2-UT3 baseline configurations). Results.The wavelength dependence of the uniform-disk diameters obtained at all epochs is characterized by a roughly constant region between 8 and 10 μm with a slight dip centered at ~9.5 μm and a gradual increase longward of 10 μm. These N-band angular sizes are significantly larger than the estimated photospheric size of V Oph. The angular sizes observed at different epochs reveal that the object appears smaller at phase 0.49 (minimum light) with uniform-disk diameters of ~5–12 mas than at phases 0.18 (~12–20 mas) and 0.65 (~9–15 mas). We interpret these results with a model consisting of optically thick C2H2 layers and an optically thin dust shell. Our modeling suggests that the C2H2 layers around V Oph are more extended (~1.7–1.8 $R_{\star}$) at phases 0.18 and 0.65 than at phase 0.49 (~1.4 $R_{\star}$) and that the C2H2 column densities appear to be the smallest at phase 0.49. We also find that the dust shell consists of amorphous carbon and SiC with an inner radius of ~2.5 $R_{\star}$, and the total optical depths of $\mbox{$\tau_{V}$} \! \approx \! 0.6$–0.9 ($\tau_{11.3~\mu{\rm m}} \! \approx \! 0.003$ and 0.004 for amorphous carbon and SiC, respectively) found at phases 0.18 and 0.65 are higher than the value obtained at phase 0.49, $\mbox{$\tau_{V}$} \! \approx \! 0.3$ ($\tau_{11.3~\mu{\rm m}} \! \approx \! 0.001$ and 0.002 for amorphous carbon and SiC, respectively). Conclusions.Our MIDI observations and modeling indicate that carbon-rich Miras also have extended layers of polyatomic molecules as previously confirmed in oxygen-rich Miras. The temporal variation of the N-band angular size is largely governed by the variations of the opacity and the geometrical extension of the C2H2 layers and the dust shell, and consequently, this masks the size variation of the photosphere. Also, the observed weakness of the mid-infrared C2H2 absorption in carbon-rich Miras can be explained by the emission from the extended C2H2 layers and the dust shell.
Abstract
Colliding stellar winds in massive binary systems have been studied through their radio, optical lines and strong X-ray emission for decades. More recently, near-infrared ...spectro-interferometric observations have become available in a few systems, but isolating the contribution from the individual stars and the wind collision region still remains a challenge. In this paper, we study the colliding wind binary γ2 Velorum and aim at identifying the wind collision zone from infrared interferometric data, which provide unique spatial information to determine the wind properties. Our analysis is based on multi-epoch Very Large Telescope Interferometer/Astronomical Multi-BEam Recombiner (VLTI/AMBER) data that allows us to separate the spectral components of both stars. First, we determine the astrometric solution of the binary and confirm previous distance measurements. We then analyse the spectra of the individual stars, showing that the O star spectrum is peculiar within its class. Then, we perform three-dimensional hydrodynamic simulations of the system from which we extract model images, visibility curves and closure phases that can be directly compared with the observed data. The hydrodynamic simulations reveal the 3D spiral structure of the wind collision region, which results in phase-dependent emission maps. Our model visibility curves and closure phases provide a good match when the wind collision region accounts for 3–10 per cent γ2 Vel's total flux in the near-infrared. The dialogue between hydrodynamic simulations, radiative transfer models and observations allows us to fully exploit the observations. Similar efforts will be crucial to study circumstellar environments with the new generation of VLTI instruments like GRAVITY and MATISSE.
Aims. We study the AU-scale circumstellar environment of the unclassified Be star V921 Sco in the near-infrared. For interpreting the observations, we employ temperature-gradient disk models. ...Methods. Using the near-infrared beam combiner instrument AMBER, we recorded spectrally dispersed (spectral resolution R = 35) interferograms in the H and K bands. To obtain an improved calibration of the visibilities, we developed a method that is able to equalize the histograms of the optical path difference of target and calibrator. We fit temperature-gradient disk models to the visibilities and spectral energy distribution (SED) to analyze the circumstellar dust geometry. Results. We derived a geometric ring-fit radius of 2.10 ± 0.16 mas in the K band. If we adopt the distance of 1150 ± 150 pc reported elsewhere, we obtain a ring-fit radius of 2.4 AU, which is slightly smaller than the 3.5 AU dust sublimation radius predicted by the size-luminosity relation. The fitted H-band radius of 1.61 ± 0.23 mas (1.85 AU) is found to be more compact than the K-band radius. The best-fit temperature-gradient disk model has an inner disk radius of ~1.45 AU, an inner-edge disk temperature T0 = 1533 K, and a temperature-gradient exponent q = 0.46 suggesting a flared disk geometry. Conclusions. The distance and luminosity of V921 Sco are not well known. If we assume a distance of 1150 ± 150 pc, we derive a ring-fit radius of ~2.4 AU, which is approximately consistent with the computed temperature-gradient disk model with inner and outer ring radii of 1.45 and 8.5 AU, respectively. If the inner radius of V921 Sco is more compact than the sublimation radius, this compact observed size can be explained by emitting material (e.g., a gaseous disk) inside the dust sublimation radius, as suggested for several other Be stars.
Aims. We present mid-IR spectro-interferometry of the Seyfert type 1 nucleus of NGC 3783. The dusty circumnuclear environment is spatially resolved and the wavelength dependence of the compact ...emission is discussed. Methods. The observations were carried out with the MIDI instrument at the Very Large Telescope Interferometer in the N-band. Spectra and visibilities were derived with a spectral resolution of $\lambda/\Delta\lambda$ ~ 30 in the wavelength range from 8 to 13 μm. For the interpretation we developed a simple dusty disk model with a small and variable covering factor. Results. At baselines of 65 and 69 m, visibilities in the range of 0.4 to 0.7 were measured. The N-band spectra show a monotonic increase of the measured flux with wavelength with no apparent silicate feature around 10 μm. We find that the mid-IR emission from the nucleus can be reproduced by an extended dust disk or torus with a small covering factor of the radiating dust clouds. Conclusions. Our mid-IR observations of NGC 3783 are consistent with a clumpy circumnuclear dust environment. The interpretation in terms of a dusty torus with a low covering factor supports a clumpy version of the unified scheme for AGN. The inferred sizes and luminosities are in good agreement with dust reverberation sizes and bolometric luminosities from optical and X-ray observations.
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