SCIENTIFIC ABSTRACT

The short-lived evolutionary phase of low and intermediate mass stars
between the Asymptotic Giant Branch (AGB) and the Planetary Nebula (PN) stage
is not well understood. The central star rapidly increases its effective
temperature and decreases its radius at constant luminosity, while the
mass loss rate, which was very high on the AGB, drops to much lower
values. As the AGB remnant expands and cools, the central star becomes
visible again and can be studied in detail.

During the transition to the PN phase, the character of the stellar wind
of the central star changes from a dust/pulsation driven one on the AGB,
to a fast, radiation driven wind once the star reaches effective
temperatures above about 20,000 K. The physics of this process is not
well known, and may be related to the geometry of the outflow. It is by
now well established that many transition objects show considerable
deviations from spherical symmetry. These asymmetries may occur shortly
before leaving the AGB, or result from the rapid evolution in this
transition phase. It is unlikely that these asymmetries arise as a
result of interacting winds, since the fast wind begins at much higher
Teff.

Another unknown is the post-AGB mass loss rate, which however is of
considerable importance for the further evolution of the central star
and nebula. If the mass loss rate exceeds the nuclear burning rate, then
mass loss determines the speed of evolution towards the PN phase, much
as mass loss dominates the AGB evolution. Estimates of post-AGB mass
loss rates range between 1e(-9) to 1e(-6) M(sun)/yr, while nuclear
burning rates range between 1e(-8) and 1e(-7) M(sun)/yr. If mass loss is
important, it speeds up the post-AGB evolution and may allow the
formation of low luminosity planetary nebulae. The infrared HI
recombination lines allow us to determine the amount of ionized
circumstellar gas and from that the post-AGB mass loss rates.

Study of the AGB remnant yields information on the mass loss history of
the star while it was on the AGB. When the energy distribution is
accuraty known, it can be modeled in detail and a kinematic age of the AGB
remnant can be determined. This age can be compared to the evolutionary
timescale of the central star, i.e. the time it took the star to reach
its present Teff. These data are critical tests of current evolutionary
tracks for post-AGB stars, especially when combined with optical and UV
abundance studies of the stellar photosphere.
Post-AGB stars also allow us to study the changes in the dust properties
as the temperature of the central star changes from very low to  very
high values. In several post-AGB stars the unidentified 21 mu feature is
seen, as well as the PAH features at 3.3, 7.7, 8.6, and 11.3 mu.

It is clear that post-AGB stars can be studied best in the (far)-IR.
ISO is the only instrument that allows a study of the IR properties of
the ionized and molecular outflow of the star, and of the properties of
the circumstellar dust.

OBSERVATION SUMMARY.

We propose to obtain SWS full scans at reduced resolution of a sample
of post-AGB stars with a range in chemical composition, luminosity,
temperature and geometry. Sample selection is mainly based on IRAS data
and follow-up observations. In addition, we propose to obtain PHT-S
spectra of all our targets. For several stars we request full resolution
SWS scans (SWS AOT01).