* Astronomy

Title: CO observations of water-maser post-AGB stars and detection of a high-velocity outflow in IRAS 15452-5459
Authors: L. Cerrigone (1), K. M. Menten (2), T. Kaminski (2) ((1) Centro de Astrobiologia, CSIC-INTA, Torrejon de Ardoz, Spain, (2) Max-Planck-Institut fuer Radioastronomie, Bonn, Germany))

Many aspects of the evolutionary phase in which Asymptotic Giant Branch stars (AGB stars) are in transition to become Planetary Nebulae (PNe) are still poorly understood. An important question is how the circumstellar envelopes of AGB stars switch from spherical symmetry to the axially symmetric structures frequently observed in PNe. In many cases there is clear evidence that the shaping of the circumstellar envelopes of PNe is linked to the formation of jets/collimated winds and their interaction with the remnant AGB envelope. Because of the short evolutionary time, objects in this phase are rare, but their identification provides valuable probes for testing evolutionary models. We have observed (sub)millimetre CO rotational transitions with the APEX telescope 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 the presence of winds traced by masers, with velocities larger than observed during that phase. We have carried out observations of several CO lines, ranging from J=2-1 up to J=7-6. In IRAS 15452-5459 we detect a fast molecular outflow in the central region of the nebula and estimate a mass-loss rate between 1.2x10^{-4} solar masses yr^{-1} (assuming optically thin emission) and 4.9x10^{-4} solar masses yr^{-1} (optically thick emission). We model the SED of this target taking advantage of our continuum measurement at 345 GHz to constrain the emission at long wavelengths. For a distance of 2.5 kpc, we estimate a luminosity of 8000 Lsun and a dust mass of 0.01 solar masses. Through the flux in the [CII] line (158 um), we calculate a total mass of about 12 solar masses for the circumstellar envelope, but the line is likely affected by interstellar contamination.

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