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

HARDI: A high angular resolution deployable interferometer for space

We describe here a proposed orbiting interferometer covering the UV, visible, and near-IR spectral ranges. With a 6-m baseline and a collecting area equivalent to about a 1.4 m diameter full aperture, this instrument will offer significant improvements in resolution over the Hubble Space Telescope, and complement the new generation of ground-based interferometers with much better limiting magnitude and spectral coverage. On the other hand, it has been designed as a considerably less ambitious project (one launch) than other current proposals. We believe that this concept is feasible given current technological capabilities, yet would serve to prove the concepts necessary for the much larger systems that must eventually be flown. The interferometer is of the Fizeau type. It therefore has a much larger field (for guiding) better UV throughout (only 4 surfaces) than phased arrays. Optimize aperture configurations and ideas for the cophasing and coalignment system are presented. The interferometer would be placed in a geosynchronous or sunsynchronous orbit to minimize thermal and mechanical disturbances and to maximize observing efficiency

Properties of the close binary and circumbinary torus of the Red Rectangle

New diffraction-limited speckle images of the Red Rectangle in the wavelength range 2.1--3.3 microns with angular resolutions of 44--68 mas and previous speckle images at 0.7--2.2 microns revealed well-resolved bright bipolar outflow lobes and long X-shaped spikes originating deep inside the outflow cavities. This set of high-resolution images stimulated us to reanalyze all infrared observations of the Red Rectangle using our two-dimensional radiative transfer code. The new detailed modeling, together with estimates of the interstellar extinction in the direction of the Red Rectangle enabled us to more accurately determine one of the key parameters, the distance D=710 pc with model uncertainties of 70 pc, which is twice as far as the commonly used estimate of 330 pc. The central binary is surrounded by a compact, massive (M=1.2 Msun), very dense dusty torus with hydrogen densities reaching n_H=2.5x10^12 cm^-3 (dust-to-gas mass ratio rho_d/rho~0.01). The bright component of the spectroscopic binary HD 44179 is a post-AGB star with mass M*=0.57 Msun, luminosity L*=6000 Lsun, and effective temperature T*=7750 K. Based on the orbital elements of the binary, we identify its invisible component with a helium white dwarf with Mwd~0.35 Msun, Lwd~100 Lsun, and Twd~6x10^4 K. The hot white dwarf ionizes the low-density bipolar outflow cavities inside the dense torus, producing a small HII region observed at radio wavelengths. We propose an evolutionary scenario for the formation of the Red Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components at a separation of 130 Rsun. The nebula was formed in the ejection of a common envelope after Roche lobe overflow by the present post-AGB star.Comment: 20 pages, 10 figures, accepted by Astronomy and Astrophysics, also available at http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.htm

Gas dynamics in the inner few AU around the Herbig B[e] star MWC297: Indications of a disk wind from kinematic modeling and velocity-resolved interferometric imaging

We present near-infrared AMBER (R = 12, 000) and CRIRES (R = 100, 000) observations of the Herbig B[e] star MWC297 in the hydrogen Br-gamma-line. Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture synthesis imaging. We interpret our velocity-resolved images as well as the derived two-dimensional photocenter displacement vectors, and fit kinematic models to our visibility and phase data in order to constrain the gas velocity field on sub-AU scales. The measured continuum visibilities constrain the orientation of the near-infrared-emitting dust disk, where we determine that the disk major axis is oriented along a position angle of 99.6 +/- 4.8 degrees. The near-infrared continuum emission is 3.6 times more compact than the expected dust-sublimation radius, possibly indicating the presence of highly refractory dust grains or optically thick gas emission in the inner disk. Our velocity-resolved channel maps and moment maps reveal the motion of the Br-gamma-emitting gas in six velocity channels, marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged. We find a rotation-dominated velocity field, where the blue- and red-shifted emissions are displaced along a position angle of 24 +/- 3 degrees and the approaching part of the disk is offset west of the star. The visibility drop in the line as well as the strong non-zero phase signals can be modeled reasonably well assuming a Keplerian velocity field, although this model is not able to explain the 3 sigma difference that we measure between the position angle of the line photocenters and the position angle of the dust disk. We find that the fit can be improved by adding an outflowing component to the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.Comment: 15 pages, 13 Figure

On the nature of the Herbig B[e] star binary system V921 Scorpii: Geometry and kinematics of the circumprimary disk on sub-AU scales

V921 Scorpii is a close binary system (separation 0.025") showing the B[e]-phenomenon. The system is surrounded by an enigmatic bipolar nebula, which might have been shaped by episodic mass-loss events, possibly triggered by dynamical interactions between the companion and the circumprimary disk (Kraus et al. 2012a). In this paper, we investigate the spatial structure and kinematics of the circumprimary disk, with the aim to obtain new insights into the still strongly debated evolutionary stage. For this purpose, we combine, for the first time, infrared spectro-interferometry (VLTI/AMBER, R=12,000) and spectro-astrometry (VLT/CRIRES, R=100,000), which allows us to study the AU-scale distribution of circumstellar gas and dust with an unprecedented velocity resolution of 3 km*s^-1. Using a model-independent photocenter analysis technique, we find that the Br-gamma-line emission rotates in the same plane as the dust disk. We can reproduce the wavelength-differential visibilities and phases and the double-peaked line profile using a Keplerian-rotating disk model. The derived mass of the central star is 5.4+/-0.4 M_sun*(d/1150 pc), which is considerably lower than expected from the spectral classification, suggesting that V921 Sco might be more distant (d approx 2kpc) than commonly assumed. Using the geometric information provided by our Br-gamma spectro-interferometric data and Paschen, Brackett, and Pfund line decrement measurements in 61 hydrogen recombination line transitions, we derive the density of the line-emitting gas (N_e=2...6*10^19 m^-3). Given that our measurements can be reproduced with a Keplerian velocity field without outflowing velocity component and the non-detection of age-indicating spectroscopic diagnostics, our study provides new evidence for the pre-main-sequence nature of V921 Sco.Comment: 17 pages, 11 figures, 3 tables, accepted by Ap

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

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