1998A&AS..129...45J

We report systematic observations of millimeter CO emission from a sample of 109 oxygen-rich evolved stars (AGB and supergiants), colour-selected from the IRAS Point Source Catalog (0.69<S_25µm/S_12µm<1.20). CO(1-0) has been searched with good sensitivity in 81 sources (74% of the sample). CO(1-0) is detected in 54 sources and a significant upper limit has been achieved in 27 sources. In our previous paper (Josselin et al., 1996A&A...315L..23J) we reported on the statistical results of these observations. We showed that in almost 50% of the sources, the ratio of the IRAS 60 µm flux to CO intensity, R=S₆₀/T_mb(1-0), is larger by a factor of 3 to more than 10 than what is expected according to the correlation found by Nyman et al. (1992A&AS...93..121N). Supergiants only exhibit very high values (≳200). In most cases, the observed spread in the values of this ratio can be explained by a large range of luminosities. This leads to a new criterion to identify AGB stars: an object with R<150 must have a low mass progenitor. Here we study the correlations between R and various physical properties of the sources. Most sources with high values of R also have low galactic latitudes, small IRAS variability indices, and early spectral types (typically M1-M5). Conversely, there is no dependence on the IRAS colours, nor on the intensity of silicate 10µm emission. However, a few AGB stars exhibit large R; other factors than luminosity are required to explain these values. Different hypotheses, such as the possible presence of a chromosphere, a low ¹²C abundance or a variable mass-loss rate, are examined. Considering the global high OH detection rate (∼67%), we studied the correlations with CO and OH emission. The detection of OH seems to be a useful discriminator of mechanisms that enhance R.