The following document lists the file abstract/DENGELS_JUNGEPPN.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
We selected 8 PPN candidates, which have probably terminated their lifetime on the AGB very recently. Unlike ordinary PPN, which already reveal optically their central stars with tempe- ratures substantially hotter then on the AGB, these 'young PPN' resemble extreme OH/IR stars, which display almost no optical and weak NIR emission. They differ from ordinary OH/IR stars by their non-variability in the NIR and by peculiar maser emission properties. The current picture of PPN evolution involves the presence of a remnant circumstellar shell left over from the AGB phase and the start of a new faster wind probably of bipolar shape. Masers may reside either in the shell or in the bipolar outflow. There is also some evidence that the chemistry of the new and old wind can differ. It is proposed to make a full grating scan of the young PPN to access their overall infrared properties, as these were not observable in the past because of the extreme redness of the objects. In the ISO data evidence for the presence of features typically displayed by advanced PPN (dust features, fast bipolar wind) will be searched for, to constrain better the evolutionary stage, in which particular features show-up during the transition phase between AGB and PN stage. The selected sample contains three objects with peculiar water maser velocities, which might indicate that they already developed a fast bipolar wind. For the spring launch window similar objects are not available. We propose to observe the evolved PPN AI Cmi (07331+0021), which shows maser emission typical for a slow radial symmetric wind. This star should be compared to objects of the "Guaranteed Time Program", which display bipolar outflows.