2008ApJ...677..382S

The results of a Hubble Space Telescope (HST) snapshot survey of post-AGB objects are shown. The aim of the survey is to complement existing HST images of protoplanetary nebulae and to connect various types of nebulosities with the physical and chemical properties of their central stars. Nebulosities are detected in 15 of 33 sources. Images and photometric and geometric measurements are presented. For sources with nebulosities we see a morphological bifurcation into two groups, DUPLEX and SOLE, as previous studies have found. We find further support for the previous results, suggesting that this dichotomy is caused by a difference in the optical thickness of the dust shell. The remaining 18 sources are classified as stellar post-AGB objects, because our observations indicate a lack of nebulosity. We show that some stellar sources may in fact be DUPLEX or SOLE objects based on their infrared colors. The causes of the differences among the groups are investigated. We discuss some evidence suggesting that high progenitor mass AGB stars tend to become DUPLEX post-AGB objects and intermediate progenitor mass AGB stars tend to become SOLE post-AGB objects. Most of the stellar sources probably have low-mass progenitors and do not seem to develop nebulosities during the post-AGB phase; therefore, they do not become planetary nebulae.