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Many aspects of the evolutionary phase in which Asymptotic Giant Branch stars(AGB stars) are in transition to become Planetary Nebulae (PNe) are stillpoorly understood. An important question is how the circumstellar envelopes ofAGB stars switch from spherical symmetry to the axially symmetric structuresfrequently observed in PNe. In many cases there is clear evidence that theshaping of the circumstellar envelopes of PNe is linked to the formation ofjets/collimated winds and their interaction with the remnant AGB envelope.Because of the short evolutionary time, objects in this phase are rare, buttheir identification provides valuable probes for testing evolutionary models.We have observed (sub)millimeter CO rotational transitions with the APEXtelescope in a small sample of stars hosting high-velocity OH and water masers.These targets are supposed to have recently left the AGB, as indicated by thepresence of winds traced by masers, with velocities larger than observed duringthat phase. We have carried out observations of several CO lines, ranging fromJ=2-1 up to J=7-6. In IRAS 15452-5459 we detect a fast molecular outflow in thecentral region of the nebula and estimate a mass-loss rate between 1.2x10^{-4}Msun yr^{-1} (assuming optically thin emission) and 4.9x10^{-4} Msun yr^{-1}(optically thick emission). We model the SED of this target taking advantage ofour continuum measurement at 345 GHz to constrain the emission at longwavelengths. For a distance of 2.5 kpc, we estimate a luminosity of 8000 Lsunand a dust mass of 0.01 Msun. Through the flux in the [CII] line (158 um), wecalculate a total mass of about 12 Msun for the circumstellar envelope, but theline is likely affected by interstellar contamination.

CO observations of water-maser post-AGB stars and detection of a high-velocity outflow in IRAS 15452-5459