We propose to study the mass loss phenomenon during the final phases of the
Asymptotic Giant Branch (AGB) evolution. To resolve the mass loss history
over the longest possible look-back times we need to map the cold (20-30 K)
and faint outer parts of the dust shells with the best possible
sensitivity. This cannot be done from the ground; the IRAS data base and
the Kuiper Airborne Observatory do not have sufficient sensitivity. For our
purposes ISOPHOT-C is the instrument of choice.

We will make very deep images at 90 and 160 microns of the extended dust
shells around obscured AGB stars and young post-AGB stars using oversampled
raster maps (AOT PHT32). For sources at a typical distance of 0.5 kpc, the
spatial resolution and sensitivity will be sufficient to study in detail
the history of the AGB mass loss with a time resolution of 3,000 yr and a
look-back time of 1E5 yr (~1-2 thermal pulse cycles). In addition, the
integrated dust distributions of the shells will provide much improved
limits on their dust masses, a property which is much debated and poorly
known. Finally, the square maps will enable us to observe the dust shell
morphology on the largest scales.