The following document lists the file abstract/RWATERS_POSTAGB.abs
from catalogue VI/111.
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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).