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Astron. Astrophys. 336, 654-661 (1998)

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Irregular variables of type Lb

Circumstellar CO emission of an oxygen-rich sample *

F. Kerschbaum 1, 2 and H. Olofsson 2

1 Institut für Astronomie, Türkenschanzstrasse 17, A-1180 Wien, Austria
2 Stockholms Observatorium, S-13336 Saltsjöbaden, Sweden

Received 4 March 1998 / Accepted 11 May 1998

Abstract

We report on the first systematic study of the mass-loss properties of O-rich irregular variables of type Lb. 12CO (J=1-0, 2-1, and 3-2) line observations were carried out using the SEST, the Onsala 20 m TELESCOPE, the JCMT and the IRAM 30 m ANTENNA. In total 20 stars were detected among the 31 objects observed (all are new detections; in three cases detections are precluded because of strong interference from interstellar CO emission). Hence, the detection rate is very high for this sample selected on the basis of the 60 µm flux strength. The majority of the detected objects are weak in CO, and the inferred mass-loss rates are low ([FORMULA]). They have, with only a few exceptions, envelopes with small gas expansion velocities (the mean value is [FORMULA]8 km/s). We find that their mass-loss 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.

Key words: stars: variables – stars: AGB – stars: circumstellar matter – stars: mass-loss – radio lines: stars

* Based on observations collected at the European Southern Observatory, La Silla, Chile, the Onsala Space Observatory, Chalmers Tekniska Högskola, Sweden, the James Clerk Maxwell Telescope, Hawaii and the IRAM 30 m telescope, Pico Veleta, Spain.

Send offprint requests to: F. Kerschbaum, Internet: kerschbaum@astro.univie.ac.at

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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