CO observations of water-maser post-asymptotic giant branch stars and detection of a high-velocity outflow in IRAS 15452–5459

¹ Centro de Astrobiología, INTA-CSIC, ctra de Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
e-mail: cerrigone@cab.inta-csic.es
² Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany

Received: 19 December 2011
Accepted: 21 March 2012

Abstract

Many aspects of the evolutionary phase in which asymptotic giant branch stars (AGB stars) are in transition to become planetary nebulae (PNe) remain 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 timescale of this evolutionary phase, objects are rare, but their identification provides valuable probes for testing evolutionary models. We observed (sub)millimeter CO rotational transitions with the APEX telescope in a small sample of stars hosting high-velocity OH and water masers. These targets are assumed 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 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 1.2 × 10^-4 M_⊙ yr^-1 (assuming optically thin emission) and 4.9 × 10^-4   M_⊙ 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 L_⊙ and a dust mass of  ~0.01 M_⊙. Through the flux in the  [CII]  line (158 μm), we calculate a total mass of about 12 M_⊙ for the circumstellar envelope, but the line is likely affected by interstellar contamination.