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Astron. Astrophys. 346, 505-519 (1999)


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High-resolution speckle masking interferometry and radiative transfer modeling of the oxygen-rich AGB star AFGL 2290 *

A. Gauger 1, Y.Y. Balega 2, P. Irrgang 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, Karachai-Cherkesia, 357147, Russia

Received 28 September 1998 / Accepted 8 March 1999

Abstract

We present the first diffraction-limited speckle masking observations of the oxygen-rich AGB star AFGL 2290. The speckle interferograms were recorded with the Russian 6 m SAO telescope. At the wavelength [FORMULA] a resolution of 75 milli-arcsec (mas) was obtained. The reconstructed diffraction-limited image reveals that the circumstellar dust shell (CDS) of AFGL 2290 is at least slightly non-spherical. The visibility function shows that the stellar contribution to the total [FORMULA] flux is less than [FORMULA], indicating a rather large optical depth of the circumstellar dust shell. The 2-dimensional Gaussian visibility fit yields a diameter of AFGL 2290 at [FORMULA] of 43 mas[FORMULA]51 mas, which corresponds to a diameter of 42 AU[FORMULA]50 AU for an adopted distance of 0.98 kpc.

Our new observational results provide additional constraints on the CDS of AFGL 2290, which supplement the information from the spectral energy distribution (SED). To determine the structure and the properties of the CDS we have performed radiative transfer calculations for spherically symmetric dust shell models. The observed SED approximately at phase 0.2 can be well reproduced at all wavelengths by a model with [FORMULA], a dust temperature of 800 K at the inner boundary [FORMULA], an optical depth [FORMULA] and a radius for the single-sized grains of [FORMULA]. However, the [FORMULA] visibility of the model does not match the observation.

Exploring the parameter space, we found that grain size is the key parameter in achieving a fit of the observed visibility while retaining the match of the SED, at least partially. Both the slope and the curvature of the visibility strongly constrain the possible grain radii. On the other hand, the SED at longer wavelengths, the silicate feature in particular, determines the dust mass loss rate and, thereby, restricts the possible optical depths of the model. With a larger grain size of [FORMULA] and a higher [FORMULA], the observed visibility can be reproduced preserving the match of the SED at longer wavelengths. Nevertheless, the model shows a deficiency of flux at short wavelengths, which is attributed to the model assumption of a spherically symmetric dust distribution, whereas the actual structure of the CDS around AFGL 2290 is in fact non-spherical. Our study demonstrates the possible limitations of dust shell models which are constrained solely by the spectral energy distribution, and emphasizes the importance of high spatial resolution observations for the determination of the structure and the properties of circumstellar dust shells around evolved stars.

Key words: stars: imaging – stars: individual: AFGL 2290 – stars: AGB and post-AGB – stars: mass-loss – stars: circumstellar matter – infrared: stars

* Based on data collected at the 6 m telescope of the Special Astrophysical Observatory in Russia

Send offprint requests to: R. Osterbart (osterbart@mpifr-bonn.mpg.de)

© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999

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