Abstract
The post asymptotic giant branch (post-AGB) phase is a short episode in the life of an intermediate-mass star. A rapid change in many physical properties of a star is suggested to happen during this phase, for example the onset of jets. However, a lot of details are still unknown. In this talk, three major problems are addressed: insufficient samples of post-AGB stars, identification of post-AGB stars, and the true status of a special class of objects called the ''water fountains (WFs)''. For the first two problems, new colour criteria for searching post-AGB stars and WFs are introduced with two maser surveys.
In the first survey which focused only on H2O masers, over 200 colour-selected AGB or post-AGB star candidates were observed. In particular, four characteristic maser sources were found, and they are currently suggested as possible very young post-AGB stars. In the second survey, another 100 objects were observed in OH and/or H2O masers. Three possible high velocity objects were discovered, including a new rare member of WFs. The colour criteria are proved to be quite sensitive in distinguishing post-AGB stars from AGB stars or other types of objects, even though there are still some contamination from young stellar objects. Regarding the third problem, one-dimensional radiative transfer models are used to study the spectral energy distributions (SEDs) of all known WFs. It has been suggested that WFs represent the very early post-AGB phase. However, it is shown from the present SED study that WFs do not necessary belong to the very early post-AGB phase, and the present interpretation on the WF status may not be entirely correct.