OBSERVED RELATIONSHIP BETWEEN CO 2-1 AND DUST EMISSION DURING THE POST-ASYMPTOTIC-GIANT-BRANCH PHASE

A CO 2-1 line survey is performed toward a sample of 58 high Galactic latitude post-asymptotic-giant-branch (pAGB) stars. To complement the observations, a compilation of literature CO 2-1 line data of known pAGB stars is done. After combining the data sets, CO 2-1 line data are available for 133 pAGB stars (about 34% of known pAGB stars) among which 44 are detections. The CO line strengths are compared with infrared dust emission for these pAGB stars by defining a ratio between the integrated CO 2-1 line flux and IRAS 25 mu m flux density (CO-IR ratio). We refer to the relationship between the CO-IR ratio and the IRAS color C23 (defined with the 25 and 60 mu m flux densities) as the CO-IR diagram. The pAGB objects are found to be located between AGB stars and planetary nebulae (PNe), and are segregated into three distinctive groups (I, II, and III) on the CO-IR diagram. By analyzing their various properties such as chemical types, spectral types, binarity, circumstellar envelope expansion velocities, and pAGB sub-types on the CO-IR diagram, we argue that the group-I objects are mainly intermediate-mass C-rich pAGB stars in the early pAGB stage (almost all of the considered carbon-rich "21 mu m" stars belong to this group), the group-II objects are massive or intermediate mass pAGB stars that already follow the profound trend of PNe, and the group-III objects are mainly low-mass binary pAGB stars with very weak CO 2-1 line emission (almost all of the considered RV Tau variables belong to this group). The CO-IR diagram is proven to be a powerful tool for investigating the co-evolution of circumstellar gas and dust during the short pAGB stage of stellar evolution.