| Description |
Although the massive winds of AGB stars are reasonably well understoodas resulting from radiation pressure on dust grains, our knowledge ofhow the mass-loss rates change as stars ascend the AGB is very poor. Asmall number of carbon-rich AGB stars have been found with detachedcircumstellar shells that imply that their mass-loss rates droppeddramatically a few thousand years ago. This decrease has beenhypothesised to result from a He-shell flash which is believed to occurperiodically in these stars; the nucleosynthesis of carbon in thisflash, and its subsequent dredge-up to the stellar surface, convertsoxygen-rich stars to carbon-rich ones. We propose to use Herschels unprecedented far-IR sensitivity andangular resolution to make a systematic search for signatures ofinterrupted mass-loss such as detached shells in a list of 21 targetswhich includes C-rich, O-rich and S-type stars (C/O~1) to test thishypothesis. Our list has been constructed using the IRAS point-sourcecatalog to identify objects which have 60-micron excesses: their 60to 25 micron flux ratio is > 0.4 and thus significantly larger than theaverage ratio for AGB stars. This excess implies the presence of acold, extended dust shell, and relatively little hot dust close to thestar, as compared to the average mass-losing AGB star. The morphologyof detached shells provides a clear indication as to its origin(circular rings due to interrupted mass-loss, paraboloidal bow-shockshaoes due to interaction of the AGB wind with the ISM): hence ourproposed PACS observations will thus be able to distinguish betweendetached shells due to interrupted mass-loss and ISM interactions. |
