摘要：Context. We follow-up on a previous finding that AGB Mira variables containing the third dredge-up indicator technetium (Tc) in their atmosphere form a different sequence ofK − [22] colour as a function of pulsation period than Miras without Tc. A near- to mid-infrared colour such asK − [22] is a good probe for thedustmass-loss rate of the stars. Contrary to what might be expected, Tc-poor Miras showredder K − [22] colours (i.e. higher dust mass-loss rates) than Tc-rich Miras at a given period.Aims. Here, the previous sample is extended and the analysis is expanded towards other colours and dust spectra. The most important aim is to investigate if the same two sequences can be revealed in thegasmass-loss rate.Methods. We analysed new optical spectra and expanded the sample by including more stars from the literature. Near- and mid-IR photometry and ISO dust spectra of our stars were investigated where available. Literature data of gas mass-loss rates of Miras and semi-regular variables were collected and analysed.Results. Our results show that Tc-poor Miras are redder than Tc-rich Miras in a broad range of the mid-IR, suggesting that the previous finding based on theK − [22] colour is not due to a specific dust feature in the 22μm band. We establish a linear relation betweenK − [22] and the gas mass-loss rate. We also find that the 13μm feature disappears aboveK − [22]≃2.17 mag, corresponding toṀg∼ 2.6 × 10−7M⊙yr−1. No similar sequences of Tc-poor and Tc-rich Miras in the gas mass-loss rate vs. period diagram are found, most probably owing to limitations in the available data.Conclusions. Different hypotheses to explain the observation of two sequences in thePvs.K − [22] diagram are discussed and tested, but so far, none of them convincingly explains the observations. Nevertheless, we might have found an hitherto unknown but potentially important process influencing mass loss on the TP-AGB.
关键词：enstars: AGB and post-AGBstars: late-typestars: evolutionstars: mass-lossstars: oscillations
