Imaging the outward motions of clumpy dust clouds around the red supergiant Antares with VLT/VISIR

We present a 0.5"-resolution diffraction-limited 17.7 micron image of the red supergiant Antares obtained with the VLT mid-infrared instrument VISIR. The VISIR image shows six clumpy dust clouds located at 0.8"--1.8" (43--96 stellar radii = 136--306 AU) away from the star. We also detected compact emission within a radius of 0.5" from the star. Comparison of our VISIR image taken in 2010 and the 20.8 micron image taken in 1998 with the Keck telescope reveals the outward motions of four dust clumps. The proper motions of these dust clumps amount to 0.2"--0.6" in 12 years. This translates into expansion velocities (projected onto the plane of the sky) of 13--40 km/s with an uncertainty of +/-7 km/s. The velocities of the dust clumps cannot be explained by a simple accelerating outflow, implying the possible random nature of the dust cloud ejection mechanism. The inner compact emission seen in the 2010 VISIR image is presumably newly formed dust, because it is not detected in the image taken in 1998. The mass of the dust clouds is estimated to be (3-6)x10^{-9} Msun. These values are lower by a factor of 3--7 than the amount of dust ejected in one year estimated from the (gas+dust) mass-loss rate of 2x10^{-6} Msun/yr, suggesting that the continuous mass loss is superimposed on the clumpy dust cloud ejection.Comment: 10 pages, 8 figures, accepted for publication in Astronomy and Astrophysic

High spectral resolution spectroscopy of the SiO fundamental lines in red giants and red supergiants with VLT/VISIR

We present high spectral resolution (R = 30000) spectroscopic observations of the SiO fundamental lines near 8.1 micron in 16 bright red giants and red supergiants using VLT/VISIR. Our sample consists of seven normal K--M giants, three Mira stars, three optically bright red supergiants, two dusty red supergiants, and the enigmatic object GCIRS3 near the Galactic center. We detected SiO fundamental lines in all of our program stars except for GCIRS3. The SiO lines in normal K and M giants as well as optically bright red supergiants do not show P-Cyg profiles or blueshifts, which means the absence of systematic outflows in the SiO line forming region. On the other hand, we detected P-Cyg profiles in the SiO lines in the dusty red supergiants VY CMa and VX Sgr (with the latter being a new detection), which suggest outflow velocities of 27 and 17 km/s, respectively. We derived basic stellar parameters (effective temperature, surface gravity, luminosity, and mass) for the normal K--M giants and optically bright red supergiants in our sample and compared the observed VISIR spectra with synthetic spectra predicted from MARCS photospheric models. Most of the SiO lines observed in the program stars warmer than ~3400 K are reasonably reproduced by the MARCS models, which allowed us to estimate the silicon abundance as well as the 28Si/29Si and 28Si/30Si ratios. However, we detected possible absorption excess in some SiO lines. Moreover, the SiO lines in the cooler red giants and red supergiant cannot be explained by the MARCS models at all even if the dust emission is taken into account. This disagreement may be a signature of the dense, extended molecular outer atmosphere.Comment: 13 pages, 10 figures, accepted for publication in Astronomy and Astrophysic

Spatially resolved, high-spectral resolution observation of the K giant Aldebaran in the CO first overtone lines with VLTI/AMBER

Aim: We present a high-spatial and high-spectral resolution observation of the well-studied K giant Aldebaran with AMBER at the Very Large Telescope Interferometer (VLTI). Our aim is to spatially resolve the outer atmosphere (so-called MOLsphere) in individual CO first overtone lines and derive its physical properties, which are important for understanding the mass-loss mechanism in normal (i.e., non-Mira) K--M giants. Methods: Aldebaran was observed between 2.28 and 2.31 micron with a projected baseline length of 10.4m and a spectral resolution of 12000. Results: The uniform-disk diameter observed in the CO first overtone lines is 20--35% larger than is measured in the continuum. We have also detected a signature of inhomogeneities in the CO-line-forming region on a spatial scale of ~45 mas, which is more than twice as large as the angular diameter of the star itself. While the MARCS photospheric model reproduces the observed spectrum well, the angular size in the CO lines predicted by the MARCS model is significantly smaller than observed. This is because the MARCS model with the parameters of Aldebaran has a geometrical extension of only ~2% (with respect to the stellar radius). The observed spectrum and interferometric data in the CO lines can be simultaneously reproduced by placing an additional CO layer above the MARCS photosphere. This CO layer is extended to 2.5 +/- 0.3 stellar radii with CO column densities of 5x10^{19}--2x10^{20} cm^-2 and a temperature of 1500 +/- 200 K. Conclusions: The high spectral resolution of AMBER has enabled us to spatially resolve the inhomogeneous, extended outer atmosphere (MOLsphere) in the individual CO lines for the first time in a K giant. Our modeling of the MOLsphere of Aldebaran suggests a rather small gradient in the temperature distribution above the photosphere up to 2--3 stellar radii.Comment: 9 pages, 6 figures, accepted for publication in Astronomy and Astrophysic

Clumpy dust clouds and extended atmosphere of the AGB star W Hya revealed with VLT/SPHERE-ZIMPOL and VLTI/AMBER II. Time variations between pre-maximum and minimum light

Our recent visible polarimetric images of the well-studied AGB star W Hya taken at pre-maximum light (phase 0.92) with VLT/SPHERE-ZIMPOL have revealed clumpy dust clouds close to the star at ~2 Rstar. We present second-epoch SPHERE-ZIMPOL observations of W Hya at minimum light (phase 0.54) in the continuum (645, 748, and 820 nm), in the Halpha line (656.3 nm), and in the TiO band (717 nm) as well as high-spectral resolution long-baseline interferometric observations in 2.3 micron CO lines with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). The high-spatial resolution polarimetric images have allowed us to detect clear time variations in the clumpy dust clouds as close as 34--50~mas (1.4--2.0 Rstar) to the star. We detected the formation of a new dust cloud and the disappearance of one of the dust clouds detected at the first epoch. The Halpha and TiO emission extends to ~150 mas (~6 Rstar), and the Halpha images reveal time variations. The degree of linear polarization is higher at minimum light (13--18%) than that at pre-maximum light. The power-law-type limb-darkened disk fit to the AMBER data in the continuum results in a limb-darkened disk diameter of 49.1+/-1.5 mas and a limb-darkening parameter of 1.16+/-0.49, indicating that the atmosphere is more extended with weaker limb-darkening compared to pre-maximum light. Our Monte Carlo radiative transfer modeling suggests the predominance of small (0.1 micron) grains of Al2O3, Mg2SiO4, and MgSiO3 at minimum light, in marked contrast to the predominance of large (0.5 micron) grains at pre-maximum light. The variability phase dependence of the grain size implies that small grains might just have started to form at minimum light in the wake of a shock, while the pre-maximum light phase might have corresponded to the phase of efficient grain growth.Comment: 14 pages, 9 figures, accepted for publication in Astronomy and Astrophysic

Exploring the water and carbon monoxide shell around Betelgeuse with VLTI/AMBER

We present the results of the analysis of our recent interferometric observations of Betelgeuse, using the AMBER instrument of the VLTI. Using the medium spectral resolution mode ($R \sim 1500$) we detected the presence of the water vapour and carbon monoxide (CO) molecules in the H and K bands. We also derived the photospheric angular diameter in the continuum. By analysing the depth of the molecular lines and the interferometric visibilities, we derived the column densities of the molecules, as well as the temperature and the size of the corresponding regions in the atmosphere of Betelgeuse (the MOLsphere) using a single shell model around the photosphere. Our results confirm the findings by Perrin \et al\ (\cite{Perrin2004}) and Ohnaka \et al\ (\cite{Ohnaka2011}) that the H$_2$O and CO molecules are distributed around Betelgeuse in a MOLsphere extending to approximately 1.3 times the star's photospheric radius.Comment: Betelgeuse Workshop, November 2012, Paris. To be published in the European Astronomical Society Publications Series, 2013, Editors: Pierre Kervella, Thibaut Le Bertre \& Guy Perri

Infrared interferometric imaging of the compact dust disk around the AGB star HR3126 with the bipolar Toby Jug Nebula

The asymptotic giant branch (AGB) star HR3126, associated with the arcminute-scale bipolar Toby Jug Nebula, provides a rare opportunity to study the emergence of bipolar structures at the end of the AGB phase. We carried out long-baseline interferometric observations with AMBER and GRAVITY (2--2.45 micron) at the Very Large Telescope Interferometer, speckle interferometric observations with VLT/NACO (2.24 micron), and imaging with SPHERE-ZIMPOL (0.55 micron) and VISIR (7.9--19.5 micron). The images reconstructed in the continuum at 2.1--2.29 micron from the AMBER+GRAVITY data reveal the central star surrounded by an elliptical ring-like structure with a semimajor and semiminor axis of 5.3 and 3.5 mas, respectively. The ring is interpreted as the inner rim of an equatorial dust disk viewed from an inclination angle of ~50 degrees, and its axis is approximately aligned with the bipolar nebula. The disk is surprisingly compact, with an inner radius of a mere 3.5 Rstar (2 au). Our 2-D radiative transfer modeling shows that an optically thick flared disk with silicate grains as large as ~4 micron can reproduce the observed continuum images and the spectral energy distribution. The images obtained in the CO first overtone bands reveal elongated extended emission around the central star, suggesting the oblateness of the star's atmosphere or the presence of a CO gas disk inside the dust cavity. The object is unresolved with SPHERE-ZIMPOL, NACO, and VISIR. If the disk formed together with the bipolar nebula, the grain growth from sub-micron to a few microns should have taken place over the nebula's dynamical age of ~3900 yrs. The non-detection of a companion in the reconstructed images implies that either its 2.2 micron brightness is more than ~30 times lower than that of the red giant or it might have been shredded due to binary interaction.Comment: 24 pages, 14 figures, published in Astronomy and Astrophysic

Spatially resolving the AGB star V3 in the metal-poor globular cluster 47 Tuc with VLTI/GRAVITY

Mass loss at the asymptotic giant branch (AGB) plays an important role not only in the final fates of stars, but also in the chemical evolution of galaxies. Nevertheless, the metallicity effects on AGB mass loss are not yet fully understood. We present spatially resolved observations of an AGB star, V3, in the metal-poor globular cluster 47 Tuc (NGC 104). The AGB star 47 Tuc V3 was observed using the GRAVITY instrument at ESO's Very Large Telescope Interferometer (VLTI) at 2-2.45 micron with a projected baseline length of up to 96 m. The object 47 Tuc V3 has been spatially resolved and stands as the first to attempt to spatially resolve an individual star in a globular cluster. The uniform-disk fit to the observed data results in an angular diameter of ~0.7 mas. Our modeling of the spectral energy distribution and near-infrared interferometric GRAVITY data suggests that the observed data can be explained by an optically thin dust shell with a 0.55 micron optical depth of 0.05-0.25, consisting of metallic iron grains, likely together with effects of the extended atmosphere of the central star. The dust temperature at the inner shell boundary is 500-800 K (corresponding to 23-90 stellar radii), significantly lower than observed in nearby oxygen-rich AGB stars. Radiation pressure on small (< 0.05 micron) iron grains is not sufficient to drive stellar winds. Therefore, iron grains may grow to larger sizes, even in the metal-poor environment. Alternatively, it is possible that the observed iron grain formation is a result of the mass outflow initiated by some other mechanism(s). The sensitivity and angular resolution of VLTI provides a new window onto spatially resolving individual stars in metal-poor globular clusters. This allows us to improve subsequent studies of the metallicity dependence of dust formation and mass loss.Comment: 7 pages, 3 figures, accepted for publication in Astronomy and Astrophysic

Properties of the CO and H2_2O MOLsphere of the red supergiant Betelgeuse from VLTI/AMBER observations

Context. Betelgeuse is the closest red supergiant (RSG); therefore, it is well suited for studying the complex processes in its atmosphere that lead to the chemical enrichment of the interstellar medium. Aims. We intend to investigate the shape and composition of the close molecular layer (also known as the MOLsphere) that surrounds the star. This analysis is part of a wider program that aims at understanding the dynamics of the circumstellar envelope of Betelgeuse. Methods. On January and February 2011, Betelgeuse was observed using the Astronomical Multi-BEam combineR (AMBER) instrument of the Very Large Telescope Interferometer (VLTI) in the H and K bands. Using the medium spectral resolution of the instrument (R $\sim$ 1500), we were able to investigate the carbon monoxide band heads and the water-vapor bands. We used two different approaches to analyse our data: a model fit in both the continuum and absorption lines and then a fit with a Radiative HydroDynamics (RHD) simulation. Results. Using the continuum data, we derive a uniform disk diameter of $41.01 \pm 0.41$~mas, a power law type limb-darkened disk diameter of $42.28 \pm 0.43$~mas and a limb-darkening exponent of $0.155 \pm 0.009$. Within the absorption lines, using a single layer model, we obtain parameters of the MOLsphere. Using a RHD simulation, we unveil the convection pattern in the visibilities. Conclusions. We derived a new value of the angular diameter of Betelgeuse in the K band continuum. Our observations in the absorption lines are well reproduced by a molecular layer at 1.2 stellar radii containing both CO and H$_2$O. The visibilities at higher spatial frequencies are matching a convection pattern in a RHD simulation.Comment: 13 pages, 11 figures, accepted for publication in Astronomy & Astrophysics; Language editin

High spectral resolution imaging of the dynamical atmosphere of the red supergiant Antares in the CO first overtone lines with VLTI/AMBER

We present high spectral resolution aperture-synthesis imaging of the red supergiant Antares (alpha Sco) in individual CO first overtone lines with VLTI/AMBER. The reconstructed images reveal that the star appears differently in the blue wing, line center, and red wing and shows an asymmetrically extended component. The appearance of the star within the CO lines changes drastically within one year, implying a significant change in the velocity field in the atmosphere. Our modeling suggests an outer atmosphere (MOLsphere) extending to 1.2--1.4 stellar radii with CO column densities of (0.5--1)x10^{20} cm^{-2} and a temperature of ~2000 K. While the velocity field in 2009 is characterized by strong upwelling motions at 20--30 km/s, it changed to strong downdrafts in 2010. On the other hand, the AMBER data in the continuum show only a slight deviation from limb-darkened disks and only marginal time variations. We derive a limb-darkened disk diameter of 37.38+/-0.06 mas and a power-law-type limb-darkening parameter of (8.7+/-1.6)x10^{-2} (2009) and 37.31+/-0.09 mas and (1.5+/-0.2)x10^{-1} (2010). We also obtain Teff = 3660+/-120 K and log L/Lsun = 4.88+/-0.23, which suggests a mass of 15+/-5 Msun with an age of 11-15 Myr. This age is consistent with the recently estimated age for the Upper Scorpius OB association. The properties of the outer atmosphere of Antares are similar to those of another well-studied red supergiant, Betelgeuse. The density of the extended outer atmosphere of Antares and Betelgeuse is higher than predicted by the current 3-D convection simulations by at least six orders of magnitude, implying that convection alone cannot explain the formation of the extended outer atmosphere.Comment: 18 pages, 16 figures, accepted for publication in Astronomy and Astrophysics, short discussion on the age of Antares and the Upper Scorpius OB association added, movies of the reconstructed images available at http://www.mpifr-bonn.mpg.de/staff/kohnaka