Abstract
We present the results of a 22GHz H2O maser survey toward a new sample of AGB
and post-AGB star candidates. Most of the objects are selected for the first
time based on the AKARI data, which have high flux sensitivity in the
mid-infrared ranges. We aim at finding H2O maser sources in the transient phase
between the AGB and post-AGB stage of evolution, where the envelopes start to
develop large deviations from spherical symmetry. The observations were carried
out with the Effelsberg 100m radio telescope. Among 204 observed objects, 63
detections (36 new) were obtained. We found 4 objects which may be "water
fountain" sources (IRAS 15193+3132, IRAS 18056-1514, OH 16.3-3.0, and IRAS
18455+0448). They possess an H2O maser velocity coverage much smaller than
those in other known water fountains. However, the coverage is still larger
than that of the 1612MHz OH maser. It implies that there is an outflow with a
higher velocity than the envelope expansion velocity (typically <25 km/s),
meeting the criterion of the water fountain class. We suggest that these
candidates are possibly oxygen-rich late AGB or early post-AGB stars in a stage
of evolution immediately after the spherically symmetric AGB mass-loss has
ceased.