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

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 2:11 m 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 visi- bility function shows that the stellar contribution to the total 2:11 m flux is less than 40%, indicating a rather large opti- cal depth of the circumstellar dust shell. The 2-dimensional Gaussian visibility fit yields a diameter of AFGL 2290 at 2:11 m of 43 mas51 mas, which corresponds to a diame- ter of 42 AU50 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 Te = 2000K, a dust temperature of 800 K at the inner boundary r1, an optical depth V = 100 and a radius for the single-sized grains of agr =0 :1 m. However, the 2:11 m 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 wave- lengths, 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 0:16 m and a higher V = 150, 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