Extended dust shells surrounding AGB stars revealed with ISO

We obtained high resolution far-infrared images of the circumstellar dust shells of several AGB carbon stars using ISOPHOT on board the ISO. We used the C-100 and C-200 detector arrays in PHT32 oversampling mode at 90 mum and 160 mum except for U Ant, which was observed at 60 mum and 90 mum. AFGL 3068 is very compact in both bands. R Scl is marginally resolved at 90 mum. U Ant shows a double shell structure, a compact but well resolved shell surrounded by a very extended envelope. Y CVn exhibits a very extended, hollow dust shell. We analyse the structure of the dust shells using a simple dust shell model and discuss the mass-loss behaviors, particularly in conjunction with the thermal pulses on the AGB.