In order to determine reliable mass-loss rates and to test the basic assumptions often used in the analysis of circumstellar emission (e.g., spherical symmetry, constant mass loss rate, etc.) we will perform detailed, non-LTE, radiative transfer calculations and compare them with our multi-transition CO observations. This will lead to much more reliable and consistent mass-loss rates than those presented already in SR_IV and Lb_IV for part of the objects of this catalogue. These models will be additionally constrained by observations of higher-J lines of CO as well as other molecules (e.g., thermal SiO lines) in order to determine the physical and chemical characteristics of these relatively warm envelopes. In the case of these low mass-loss rate objects, where the higher frequency of peculiar line profiles suggests a complex envelope structure, spatial information is of particular importance. The most nearby objects can be mapped in suitable molecular emissions with mm-wave interferometers.
The authors thank the referee Claudine Kahane for constructive comments on the manuscript. The work of FK was supported by APART (Austrian Programme for Advanced Research and Technology) from the Austrian Academy of Sciences. FK was also supported by the Fonds zur Förderung der wissenschaftlichen Forschung under project number S7308-AST and the Österreichische Nationalbank under the Jubiläumsfonds-project number 6876. HO acknowledges financial support from the Swedish Natural Science Research Council.
The authors are very grateful to Martin Groenewegen for providing us with all the valuable IRAM observations in this catalogue. He also contributed a few SEST spectra. Göran Sandell is thanked for carefully performing the JCMT observations, and Andreas Andersson for carefully performing the August-98 SEST observations. The reduction of the CO spectra was made using the DRP software kindly provided by Michael Olberg, OSO.
The Swedish-ESO Submillimetre Telescope, SEST, is operated jointly by ESO and the Swedish National Facility for Radio Astronomy, Onsala Space Observatory at Chalmers University of Technology. The James Clerk Maxwell Telescope is operated by The Joint Astronomy Centre on behalf of the Particle Physics and Astronomy Research Council of the United Kingdom, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada. This research has made use of the SIMBAD database.
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