Water fountains: bipolar fast stellar jets traced by water vapor maser emission

Highly collimated, bipolar fast jets are found in asymptotic giant branch (AGB) and post-AGB stars as well as in active galactic nuclei and young stellar objects. It is still unclear how to launch such jets from dying stars that were originally spherically symmetric. Exploration of the stellar jet evolution is also expected to probe its role in shaping a planetary nebula. Interestingly, some of stellar H₂O maser sources - water fountains - exhibit stellar jets with spatially and kinematically high collimation in the earliest phase (<1000 years) of the jet evolution. Such water fountains have been identified in 14 sources to date. We have recently conducted interferometric (VLBA, EVN, VERA, VLA) maser and the single-dish (ASTE) CO J = 3 -> 2 line observations of the water fountains. They have revealed a typical dynamical age (< 100 yr) and the detailed kinematical structures of the water fountains, possibility of the coexistence of ``equatorial flows'', and their locations and kinematics in the Milky Way. Based on these results, the masses and evolutionary statuses of the host stars are also estimated.