| 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_{25microns }/S_{12microns } < 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 we reported on the statistical results of these observations. We showed that in almost 50% of the sources, the ratio of the IRAS 60microns flux to CO intensity, {cal 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 \cite[Nyman et al. (1992)]{nym92}. Supergiants only exhibit very high values (raise 0.6exhbox {> 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 {cal R} < 150 must have a low mass progenitor. Here we study the correlations between {cal R} and various physical properties of the sources. Most sources with high values of {cal 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 10microns emission. However, a few AGB stars exhibit large {cal R} other factors than luminosity are required to explain these values. Different hypotheses, such as the possible presence of a chromosphere, a low (12) C 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 {cal R}. |
Target sources:
IRAS
Comments:
Velocity resolution: n.a.
Upper limit for non-detections: 0.15 Jy (3σ)
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