Morphological Classification of Post-AGB Stars

We present a complete study of the morphology of post-Asymptotic Giant Branch (post-AGB) stars. Post-AGB is a very short evolutionary phase between the end of the AGB and the beginning of the Planetary Nebula (PN) stage (between 100 and 10,000 yrs). We have defined the end of the post-AGB phase and the beginning of the PN phase as when the star becomes hot enough to fully ionize the hydrogen envelope. Post-AGB stars have a circumstellar shell that is illuminated by the central star or partially ionized. However, this circumstellar shell is too small to be resolved by ground-based observations. Thus, we have used data from the Hubble Space Telescope (HST) database to resolve these shells. About 150 post-AGB stars were found in this database. Here we present the preliminary results on their morphological classification and the correlation with several parameters such as Galactic latitude and IRAS flux. Our preliminary results show that 40 % of the sample are stellar-like (S), 33 % bipolar (B), 12 % multi-polar (M), and 15 % elliptical (E).