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A&A Supplement series, Vol. 129, April I 1998, 45-68

Received March 21; accepted September 17, 1997

Infrared giants vs. supergiants

II. CO observations

E. Josselin tex2html_wrap2809 - C. Loup tex2html_wrap2811 - A. Omont tex2html_wrap2809 - C. Barnbaum tex2html_wrap2815 - L.-Å. Nyman tex2html_wrap2817 - F. Sèvre tex2html_wrap2809

Send offprint request: E. Josselin

tex2html_wrap2821  Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago, F-75014 Paris, France
tex2html_wrap2823  STScI, 3700 San Martin Dr., Baltimore, MD 21218, U.S.A.
tex2html_wrap2825  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 tex2html_wrap_inline2769. 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 tex2html_wrap_inline2775 flux to CO intensity, tex2html_wrap_inline2777, 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 (tex2html_wrap_inline2779). 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 tex2html_wrap_inline2781 must have a low mass progenitor.

Here we study the correlations between tex2html_wrap_inline2783 and various physical properties of the sources. Most sources with high values of tex2html_wrap_inline2783 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 tex2html_wrap_inline2787 emission. However, a few AGB stars exhibit large tex2html_wrap_inline2783; 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 tex2html_wrap_inline2793, we studied the correlations with CO and OH emission. The detection of OH seems to be a useful discriminator of mechanisms that enhance tex2html_wrap_inline2783.

keywords: molecular processes -- stars: circumstellar matter -- stars: mass loss -- stars: supergiants -- radio lines: stars

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