CO OBSERVATIONS OF COLD IRAS OBJECTS - AGB AND POST-AGB STARS

A sample of 77 objects with IRAS flux ratios similar to those of evolved AGB stars with very cold circumstellar envelopes (CSE) were investigated, and CO observations were used to help identify their nature. Most of the objects are AGB stars, planetary nebulae, and objects in transition between them (post-AGB stars). The 21 Post-AGB candidates in the sample share the characteristics of low color temperature and, for most, low IRAS variability. They are discussed in terms of three partially over-lapping categories: intermediate spectral type stars with expanding molecular shells, cold circumstellar shells with CO but with no OH emission, and irregular or non-variable OH/IR stars. Sites of star formation in molecular clouds can have infrared colors similar to cold CSE, but most of the cloud sources have a higher ratio of IRAS 100-mu-m to 60-mu-m fluxes (> 0.6) than evolved stars. Narrow CO emission was detected from several cloud sources with colors similar to cold CSE (few of which are in the sample of 77 objects). Because of the selection criteria, these sources may tend to be the most deeply embedded protostars, or young stars with only a relatively small amount of material still near them.