A wall of dust around a proto-Mira

We present the discovery of a huge (19$\arcmin$ $\times$ 16$\arcmin$) dust ring surrounding a bright (V = 10.60) red star. The dust ring has, at D = 700 pc, a diameter of 4 pc, and a central hole of ~1.5 pc across. Part of the shell is also seen as an absorption nebulosity. The star is classified as a M3III AGB star. Among AGB stars its detached shell is of unrivalled size. Detached shells around AGB stars are normally interpreted in terms of thermal pulses. However, in this case a significant fraction of the shell may consist of swept-up ISM; the detached appearance can be explained with wind--ISM interaction. We present a model where the AGB wind has been stopped by the surrounding ISM, and the swept-up shell is now expanding at the sound speed. The model predicts that the ring will disperse over a few times 10^5 yr, and eventually will leave a large hole in the ISM surrounding the AGB star or its future planetary nebula.