Mapping the 12CO J=1-0 and J=2-1 emission in AGB and early post-AGB circumstellar envelopes

We present COSAS (CO Survey of late AGB Stars), a project to map and analyze the (CO)-C-12 J = 1-0 and J = 2-1 line emission in a representative sample of circumstellar envelopes around AGB and post-AGB stars. The survey was undertaken with the aim of investigating small-and large-scale morphological and kinematical properties of the molecular environment surrounding stars in the late AGB and early post-AGB phases. For this, COSAS combines the high sensitivity and spatial resolving power of the IRAM Plateau de Bure interferometer with the better capability of the IRAM 30 m telescope to map extended emission. The global sample encompasses 45 stars selected to span a range in chemical type, variability type, evolutionary state, and initial mass. COSAS provides means to quantify variations in the mass-loss rates, assess morphological and kinematical features, and to investigate the appearance of fast aspherical winds in the early post-AGB phase. This paper, which is the first of a series of COSAS papers, presents the results from the analyses of a first sample of 16 selected sources. The envelopes around late AGB stars are found to be mostly spherical, often mingled with features such as concentric arcs (RCas and TXCam), a broken spiral density pattern (TXCam), molecular patches testifying to aspherical mass-loss (WXPsc, IKTau, VCyg, and SCep), and also with well-defined axisymmetric morphologies and kinematical patterns (XHer and RXBoo). The sources span a wide range of angular sizes, from relatively compact (CRL 2362, OH 104.9+2.4 and CRL 2477) to very large (chi Cyg and TXCam) envelopes, sometimes partially obscured by self-absorption features, which particularly for IKTau and chi Cyg testifies to the emergence of aspherical winds in the innermost circumstellar regions. Strong axial structures with more or less complex morphologies are detected in four early post-AGB stars (IRAS 20028+3910, IRAS 23321+6545, IRAS 19475+3119 and IRAS 21282+5050) of the sub-sample.