IRREGULAR VARIABLES OF TYPE LB - CIRCUMSTELLAR CO EMISSION OF AN OXYGEN-RICH SAMPLE

We report on the first systematic study of the mass-loss properties of O-rich irregular variables of type Lb. (CO)-C-12 (J=1-0, 2-1 and 3-2) line observations were carried out using the SEST, the Onsala 20 m TE LESCOPE, the JCMT and the LRAM 30 m ANTENNA. In total 20 stars were de tected among the 31 objects observed (all are new detections; in three cases detections are precluded because of strong interference from in terstellar CO emission). Hence, the detection rate is very high for th is sample selected on the basis of the 60 mu m flux strength. The majo rity of the detected objects are weak in CO, and the inferred mass-los s rates are low (less than or similar to 10(-7) M./yr). They have, wit h only a few exceptions, envelopes with small gas expansion velocities (the mean value is approximate to 8 km/s). We find that their mass-lo ss properties are very similar to those of bright O-rich semiregular ( SRa or SRb) and Mira variables, i.e., at least in this mass-loss rate range the stellar mass-loss properties are not strongly influenced by the pulsational behaviour of the star.