A&A Supplement series, Vol. 129, April I 1998, 45-68
Received March 21; accepted September 17, 1997
E. Josselin 
- C. Loup 
- A. Omont
- C. Barnbaum 
- L.-Å. Nyman 
- F. Sèvre
Send offprint request: E. Josselin
Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago,
F-75014 Paris, France
STScI, 3700 San Martin Dr., Baltimore, MD 21218, U.S.A.
European Southern Observatory, Casilla 19001, Santiago 19, Chile
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
.
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 
flux to CO intensity,
,
is larger by a factor of 3 to more than 10 than what is expected
according to the correlation found by Nyman et al. (1992).
Supergiants only exhibit very high values (
).
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 
must have a low mass progenitor.
Here we study the correlations between 
and various physical
properties of the sources. Most sources with high values of
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 
emission.
However, a few AGB stars exhibit large ;
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 
, we studied the
correlations with CO and OH emission. The detection of OH seems to be a
useful discriminator of mechanisms that enhance .
keywords: molecular processes -- stars: circumstellar matter -- stars: mass loss -- stars: supergiants -- radio lines: stars