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

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7. Conclusion and outlook

We have for the first time detected a larger sample of O-rich irregularly variable red giants (variability type Lb) in circumstellar CO emission, a signpost of circumstellar envelopes produced by stellar mass-loss. The majority of the detected objects are weak in CO, and the estimated mass-loss rates are low, [FORMULA]. The envelope gas expansion velocities are, with only a few exceptions, small (the mean value is [FORMULA]8 km/s, but we find objects with velocities as low as [FORMULA]3 km/s!).

A comparison between stellar and circumstellar properties of these stars and samples of O-rich SRVs and Miras shows that, at least in the mass-loss rate range studied here, the gas expansion velocity does not depend on the stellar effective temperature, nor on the pulsational amplitude or period of the star. Likewise, the mass-loss rate distributions are very similar for these three groups of variables. Consequently, the stellar mass-loss properties seem not to be strongly influenced by the pulsational regularity or even the mode (compare SRIII and SRIV). There is a trend of increasing gas expansion velocity with the IRAS LRS-class (2n), which suggests that the expansion velocity increases along with the mass loss rate.

For the future we propose observations of higher-J lines of CO (since the line intensity increases substantially with frequency) as well as other molecules in order to determine the physical and chemical characteristics of these relatively warm envelopes. In general, this type of envelopes are not so well studied, and in a forthcoming paper we will analyze the CO and the dust emission from the envelopes of IRVs and SRVs in much more detail. The most nearby objects can be mapped in suitable molecular emissions with mm-wave interferometers. This is of special importance for these low mass-loss rate objects where the higher frequency of peculiar line profiles suggest a complex envelope structure. In order to investigate the dust mass loss properties of IRVs a small number of typical objects are observed with the Short Wave Spectrometer on board the ISO satellite.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998