The following document lists the file abstract/RSZCZERB_AGB_C_OH.abs
from catalogue VI/111.
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It is presently a widely accepted idea that single carbon stars are formed during the evolution on the AGB when carbon rich material is dredged up, probably during and/or just after a helium shell flash, from the interior of the star to its oxygen rich photosphere (Iben and Renzini 1983, Ann. Rev. Astr. Ap. 21, 271, Willems and de Jong 1988, AA 196, 173). The model of Willems and de Jong, proposed after the discovery of carbon stars with silicate dust emission, assumes that when the photosphere of the star becomes carbon rich for the first time the mass loss rate strongly decreases compared to the previous phase of evolution. In consequence, the detached oxygen rich shell will expand around the carbon star which will become very soon visible. Up to now 19 carbon stars with silicate emission are known (Kwok et al. 1997, ApJS 112, 557). However, another signature of the oxygen rich envelope, the OH maser emission, has been detected for only one of these sources. Our recent investigations of the available data from OH surveys and the published catalogue of the IRAS LRS spectra (Kwok et al. 1997) allowed us to select a NEW group of sources which could be related to the transition phase between oxygen and carbon stars. Namely, we have found 6 NEW carbon stars which also show OH maser emission at 1612 MHz. These objects could be more evolved than objects showing only silicate emission in their LRS spectra. Probably, only after short lasting interruption mass loss resume again but now with carbon based dust. We propose to obtain ISO SWS01 spectra in order to better understand the transition between oxygen and carbon rich stars and to constrain the behaviour of mass loss which leads to the so unexpected phenomenon as an OH emission around carbon star.