We have demonstrated that the occurence of high values of the far
infrared-to-CO emission (
) in circumstellar
envelopes around evolved stars (AGB stars or infrared supergiants) is not
limited to the coldest objects (Heske et al. 1990), where a low kinetic
temperature was invoked to explain the high values of .
Warmer objects, i.e., those with a lower opacity
(-0.48< C21 < -0.24) are also often found to have high values
of . is related to intrinsic parameters
of the object, such as the the luminosity and hence the initial mass. A low
value of this ratio is characteristic of a low mass star
(
). As shown in Paper I,
This represents a new tool to discriminate between AGB stars and supergiants.
However a few AGB stars have values
comparable to what is observed for supergiants. The cause is not clear, but
it seems that only the more massive stars
or peculiar objects are so affected. The mass-loss history,
the geometry of the envelope, a low abundance of CO, taken together,
are possible factors.
Acknowledgements
We want to thank the IRAM and SEST-ESO staffs for their assistance during the observations which constitute the basis of this paper. We also thank T. Forveille for his contribution to the first observations and a careful reading of this paper, and M. Groenewegen for the observation reported in Table 17 (click here) of CO(1-0) in U Equ. This research has made use of the Simbad database, operated at CDS, Strasbourg, France. The Nançay Radio Observatory is the Unité Scientifique de Nançay of the Observatoire de Paris, associated as Unité de Service et de Recherche (USR) No. B704 to the French Centre National de Recherche Scientifique (CNRS). The Nançay Observatory also gratefully acknowledges the financial support of the Conseil Régional of the Région Centre in France.