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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 2.11 micron 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 0.005 micron < a < 0.45 micron. 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 shells 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 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.
(abridged) CIT 3 is an oxygen-rich long-period variable evolving along the AGB and one of the most extreme infrared AGB objects. We present the first bispectrum speckle-interferometry observations of CIT 3 in the J-, H-, and K-band (resolution: 48mas
(abridged) NML Cyg is a highly evolved OH/IR supergiant and supposed to be among the most luminous supergiants in the galaxy. We present the first diffraction limited 2.13micron observations of NML Cyg with 73mas resolution. The speckle interferogram
Among evolved massive stars likely in transition to the Wolf-Rayet phase, IRC +10420 is probably one of the most enigmatic. It belongs to the category of yellow hypergiants and it is characterized by quite high mass loss episodes. Even though IRC +10
The circumstellar envelope of the hypergiant star IRC+10420 has been traced as far out in SiO J=2-1 as in CO J = 1-0 and CO J = 2-1, in dramatic contrast with the centrally condensed (thermal) SiO- but extended CO-emitting envelopes of giant and supe
We obtained near-infrared long-baseline interferometry of IRC+10420 with the AMBER instrument of ESOs Very Large Telescope Interferometer (VLTI) in low and high spectral resolution (HR) mode to probe the photosphere and the innermost circumstellar en