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Astron. Astrophys. 348, 805-814 (1999)

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The rapidly evolving hypergiant IRC +10 420: High-resolution bispectrum speckle-interferometry and dust-shell modelling

T. Blöcker 1, Y. Balega 2, K.-H. Hofmann 1, J. Lichtenthäler 1, R. Osterbart 1 and G. Weigelt 1

1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2 Special Astrophysical Observatory, Nizhnij Arkhyz, Zelenchuk region, 35147 Karachai-Cherkesia, Russia

Received 9 October 1998 / Accepted 4 June 1999

Abstract

The hypergiant IRC +10 420 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 1000-2000 K within only 20 yr. We present the first speckle observations of IRC +10 420 with 73 mas resolution. A diffraction-limited 2.11 µm image was reconstructed from 6 m telescope speckle data using the bispectrum speckle-interferometry method. The visibility function shows that the dust shell contributes [FORMULA] to the total flux and the unresolved central object [FORMULA].

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, [FORMULA] [FORMULA] [FORMULA], with a ranging between [FORMULA] = 0.005 µm and [FORMULA] = 0.45 µm. 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 [FORMULA] density distribution. The best model for both SED and visibility was found for a dust shell with a dust temperature of 1000 K at its inner radius of [FORMULA]. At a distance of [FORMULA] the density was enhanced by a factor of 40 and and its density exponent was changed from [FORMULA] to [FORMULA]. The shell's intensity distribution was found to be ring-like. The ring diameter is equal to the inner diameter of the hot shell ([FORMULA] mas). The diameter of the central star is [FORMULA] mas. The assumption of a hotter inner shell of 1200 K gives fits of almost comparable quality but decreases the spatial extension of both shells' inner boundaries by [FORMULA]% (with [FORMULA] 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 [FORMULA] kpc) ago. The bolometric flux, [FORMULA], is [FORMULA] Wm-2 corresponding to a central-star luminosity of [FORMULA].

Key words: stars: Wolf-Rayet – stars: supergiants – stars: mass-loss – stars: individual: IRC +10 420 – stars: circumstellar matter – techniques: image processing

Send offprint requests to: T. Blöcker (bloecker@mpifr-bonn.mpg.de)

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199
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