Circumstellar H I and CO around the carbon stars V1942 Sagittarii and V Coronae Borealis

Context. The majority of stars that leave the main sequence are undergoing extensive mass loss, in particular during the asymptotic giant branch (AGB) phase of evolution. Observations show that the rate at which this phenomenon develops differs widely from source to source, so that the time-integrated mass loss as a function of the initial conditions (mass, metallicity, etc.) and of the stage of evolution is presently not well understood. Aims. We investigate the mass loss history of AGB stars by observing the molecular and atomic emission of their circumstellar envelopes. Methods. We selected two stars that are on the thermally pulsing phase of the AGB (TP-AGB) for which high quality data in both the CO rotation lines and the atomic hydrogen line at 21 cm could be obtained. Results. A carbon star of the irregular variability type, V1942 Sgr, has a complex CO line profile that may originate in a long-lived

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