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Astron. Astrophys. 338, 132-138 (1998)

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1. Introduction

The study of the physical mechanisms by which stars, at each phase of their evolution, enrich the interstellar medium by losing their material is of high interest to stellar physics. All stars with initial mass on the main sequence between 1 to 8 [FORMULA] (solar masses) stay in the so-called Asymptotic Giant Branch (AGB) before producing a planetary nebula. This stage is characterized by an ejection of mass, continuous or violent, producing an extended circumstellar shell of dust and gas. Many questions concerning the physical mechanisms which drive the mass loss and the processes which govern the formation and the destruction of the dust grains have still no answer. A large number of observations suggest that the mass loss increases and deviates from spherical symmetry while the star evolves along the AGB. A preliminary survey has led to the result that at least half of the sources are non-spherical (Dougados 1991). A further mid-IR imaging and millimeter interferometric mapping study of more than 50 evolved stars (Meixner 1993) has also led to the conclusion that the last stage of AGB mass loss is inherently aspherical. In this context, we have proposed to obtain diffraction-limited images in the near infrared (J, H and K) of a small sample of well studied and well documented late type stars for which photometric, spectroscopic and polarimetric data are available. Our sample of objects consisted of both oxygen-rich and carbon-rich stars at various phases of their evolution and with various shell opacities. Some of them exhibit a non-spherical geometry shown by the polarization measurements, the observed bipolar structure and the difficulty to fit accurately the observed broad band spectrum with classical spherical models (Rown-Robinson & Harris 1983a, 1983b). The near-infrared domain helps to probe the hot dust component and the structure associated to the inner circumstellar envelope. The inner radius of the dust envelope is a particularly critical parameter in the dust formation modelling. The measurements obtained with the Infrared Spatial Interferometer (ISI) provide some constraints on the mass loss mechanisms (Danchi et al. 1994). Recent long baseline interferometric observations at 11µm have shown, for example, that the supposed well-known Mira star o Ceti displays a geometrical asymmetry of its shell structure (Lopez et al. 1997a). The geometrical shell parameters, the dust density law, the extinction opacity and constraints on the dust grain sizes can be deduced (Lopez et al. 1995). Presently the dust shell morphology is best studied with imaging systems rather than with 2-telescope interferometry which just leads to one-dimensional visibility curves. Although the angular resolution of a 3.60-m telescope class does not allow to resolve the central star and the inner shell, the interest of adaptive optics lies in its capability of imaging the large scale structures.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998