1) M. Wittkowski, N. Langer and G. Weigelt

Diffraction-limited speckle-masking interferometry of the red supergiant VY CMa

Astronomy and Astrophysics 340, L39-L42 (1998)
Abstract. We present the first diffraction-limited images of the mass-loss envelope of the red supergiant star VY CMa. The two-dimensional optical and NIR images were reconstructed from 3.6m telescope speckle data using bispectrum speckle interferometry. At the wavelengths 0.8 micron (RG780 filter), 1.28 micron and 2.17 micron the diffraction-limited resolutions of 46mas, 73mas, and 124mas were achieved. All images clearly show that the circumstellar envelope of VY CMa is non-spherical. The RG780, 1.28 micron, and 2.17 micron FWHM Gauss fit diameters are 67mas x 83mas, 80mas x 116mas and 138mas x 205mas, respectively, or 100 AU x 125 AU, 120 AU x 174 AU and 207 AU x 308 AU (for a distance of 1500pc). We discuss several interpretations for the asymmetric morphology. Combining recent results about the angular momentum evolution of red supergiants and their pulsational properties, we suggest that VY CMa is an immediate progenitor of IRC+10420, a post red supergiant during its transformation into a Wolf-Rayet star.
2) T. Blöcker, Y. Balega, K.-H. Hofmann, J. Lichtenthäler, R. Osterbart and G. Weigelt

The rapidly evolving hypergiant IRC+10420: High-resolution bispectrum speckle-interferometry and dust-shell modelling

Astronomy & Astrophysics 348, 805-814 (1999)

Abstract. The hypergiant IRC+10420 is a unique object for the study of stellar evolution since it is the only object that is believed to be witnessed in its rapid transition from the red supergiant stage to the Wolf-Rayet phase. Its effective temperature has increased by 1000-2000K within only 20yr. We present the first speckle observations of IRC+10420 with 73mas resolution. A diffraction-limited 2.11µm image was reconstructed from 6m telescope speckle data using the bispectrum speckle-interferometry method. The visibility function shows that the dust shell contributes ~40% to the total flux and the unresolved central object ~60%. Radiative transfer calculations have been performed to model both the spectral energy distribution and visibility function. The grain sizes, a, were found to be in accordance with a standard distribution function, n(a) ~ a^(-3.5), with a ranging between amin=0.005µm and amax=0.45µm. The observed dust shell properties cannot be fitted by single-shell models but seem to require multiple components. At a certain distance we considered an enhancement over the assumed 1/r^x density distribution. The best model for both SED and visibility was found for a dust shell with a dust temperature of 1000K at its inner radius of 69Rstar. At a distance of 308Rstar the density was enhanced by a factor of 40 and and its density exponent was changed from x=2 to x=1.7. The shell's intensity distribution was found to be ring-like. The ring diameter is equal to the inner diameter of the hot shell (69mas). The diameter of the central star is ~1mas. The assumption of a hotter inner shell of 1200K gives fits of almost comparable quality but decreases the spatial extension of both shells' inner boundaries by ~30% (with x=1.5 in the outer shell).
The two-component model can be interpreted in terms of a termination of an enhanced mass-loss phase roughly 60 to 90 yr (for d=5kpc) ago. The bolometric flux, Fbol, is 8.17x10^(-10)Wm^-2 corresponding to a central-star luminosity of L/Lsol = 25462x(d/kpc)^2.
3) T. Blöcker, Y. Balega, K.-H. Hofmann, and G. Weigelt

Bispectrum speckle interferometry observations and radiative transfer modelling of the red supergiant NML Cyg: Multiple dust-shell structures evidencing previous superwind phases

Astronomy & Astrophysics 369, 142-154 (2001)

Abstract. NML Cyg is a highly evolved OH/IR supergiant, one of the most prominent infrared objects due to its strong obscuration by dust, and supposed to be among the most luminous supergiants in the galaxy. We present the first diffraction-limited 2.13µm observations of NML Cyg with 73mas resolution. The speckle interferograms were obtained with the 6m telescope at the Special Astrophysical Observatory, and the image reconstruction is based on the bispectrum speckle-interferometry method. The visibility function declines towards the diffraction limit to ~0.6.
Radiative transfer calculations have been carried out to model the spectral energy distribution, given by ground-based photometry and ISO spectroscopy, and our 2.13µm visibility function. Additionally, mid-infrared visibility functions at 11µm were considered. The observed dust shell properties do not appear to be in accordance with standard single-shell (uniform outflow) models but seem to require multiple components. Considering previous periods of enhanced mass-loss, various density enhancements in the dust shell were taken into account. An extensive grid of models was calculated for different locations and strenghts of such superwind regions in the dust shell. To match the observations from the optical to the sub-mm domain requires at least two superwind regions embedded in the shell. The best model includes a dust shell with a temperature of 1000K at its inner radius of 6.2 Rstar, a close embedded superwind shell extending from 15.5 Rstar to 21.7 Rstar with an amplitude (factor of density enhancement) of 10, and a far-out density enhancement at 186 Rstar with an amplitude of 5. The angular diameters of the central star and of the inner rim of the dust shell amount to 16.2mas and 105mas, resp. The diameter of the embedded close superwind region extends from 263mas to 368mas, and the inner boundary of the distant superwind region has a diameter of 3.15". In the near-infrared the dust condensation zone is limb-brightened leading to a corresponding ring-like intensity distribution. The grain sizes, a, were found to be in accordance with a standard distribution function, n(a)~a^-3.5, with a ranging between amin=0.005µm and amax=0.15µm. The bolometric flux amounts to Fbol=3.63 10^-9 Wm^-2 corresponding to a central-star luminosity of L/Lsol=1.13 10^5 (d/kpc)^2. Within the various parts of the dust shell, 1/r^2 density distributions could be maintained differing only in their amplitude A. A slight improvement of the far-infrared properties can be obtained if a shallower density distribution of rho~1/r^1.7 is considered in the distant superwind region. The present-day mass-loss rate was determined to be Mdot=1.2 10^-4 Msol/yr. The inner embedded superwind shell corresponds to a phase of enhanced mass-loss (with amplitude 10) in the immediate history of NML Cyg which began ~59yr ago and lasted for ~18yr. Correspondingly, the outer superwind region is due to to a high mass-loss period (amplitude 5) which terminated 529yr ago.

4) B. Yudin, Y. Balega, T. Blöcker, K.-H. Hofmann, D. Schertl, and G. Weigelt:

Speckle interferometry and radiative transfer modelling of the Wolf-Rayet star WR 118

Astronomy and Astrophysics 379, 229-234 (2001)

Abstract. WR 118 is a highly evolved Wolf-Rayet star of the WC10 subtype surrounded by a permanent dust shell absorbing and re-emitting in the infrared a considerable fraction of the stellar luminosity. We present the first diffraction-limited 2.13µm speckle interferometric observations of WR 118 with 73 mas resolution. The speckle interferograms were obtained with the 6m telescope at the Special Astrophysical Observatory. The two-dimensional visibility function of the object does not show any significant deviation from circular symmetry. The visibility curve declines towards the diffraction cut-off frequency to ~0.66 and can be approximated by a linear function. Radiative transfer calculations have been carried out to model the spectral energy distribution, given in the range of 0.5-25µm, and our 2.13µm visibility function, assuming spherical symmetry of the dust shell. Both can be fitted with a model containing double-sized grains (``small'' and ``large'') with the radii of a = 0.05µm and 0.38µm, and a mass fraction of the large grains greater than 65%. Alternatively, a good match can be obtained with the grain size distribution function n(a)~ a-3 with a ranging between 0.005µm and 0.6µm. At the inner boundary of the modelled dust shell (angular diameter (17 +/- 1)mas), the temperature of the smallest grains and the dust shell density are (1750+/-100)K and (1 +/- 0.2) 10-19g/cm^3, respectively. The dust formation rate is found to be (1.3 +/- 0.5) 10-7 Msol/yr, assuming vwind = 1200km/s.

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