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

I. The COSAS program, first sample

¹ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38406 Saint-Martin d’Hères France
e-mail: ccarrizo@iram.fr; neri@iram.fr; winters@iram.fr; grewing@iram.fr
² Instituto de Radioastronomia Milimétrica, Avda. Divina Pastora 7, 18012 Granada, Spain
e-mail: g.quintana@iram.es
³ Observatorio Astronómico Nacional, Apartado 1143, 28800 Alcalá de Henares, Spain
e-mail: v.bujarrabal@oan.es
⁴ Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden
e-mail: schoier@chalmers.se; hans.olofsson@chalmers.se; michael.lindqvist@chalmers.se
⁵ Observatorio Astronómico Nacional (IGN), Alfonso XII No. 3, 28014 Madrid, Spain
e-mai: j.alcolea@oan.es
⁶ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Chile
e-mail: rlucas@alma.cl

Received: 8 April 2010
Accepted: 10 June 2010

Abstract

We present COSAS (CO Survey of late AGB Stars), a project to map and analyze the ¹²CO 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 (R Cas and TX Cam), a broken spiral density pattern (TX Cam), molecular patches testifying to aspherical mass-loss (WX Psc, IK Tau, V Cyg, and S Cep), and also with well-defined axisymmetric morphologies and kinematical patterns (X Her and RX Boo). 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 (χ Cyg and TX Cam) envelopes, sometimes partially obscured by self-absorption features, which particularly for IK Tau and χ 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.