| Description |
Although the massive winds of AGB stars are reasonably well understood as resulting from radiation pressure on dust grains, our knowledge of how the mass-loss rates change as stars ascend the AGB is very poor. A small number of carbon-rich AGB stars have been found with detached circumstellar shells that imply that their mass-loss rates dropped dramatically a few thousand years ago. This decrease has been hypothesised to result from a He-shell flash which is believed to occur periodically in these stars; the nucleosynthesis of carbon in this flash, and its subsequent dredge-up to the stellar surface, converts oxygen-rich stars to carbon-rich ones. We propose to use Herschel.s unprecedented far-IR sensitivity and angular resolution to make a systematic search for signatures of interrupted mass-loss such as detached shells in a list of 21 targets which includes C-rich, O-rich and S-type stars (C-Ovirgul1) to test this hypothesis. Our list has been constructed using the IRAS point-source catalog to identify objects which have 60-micron excesses: their 60 to 25 micron flux ratio is > 0.4 and thus significantly larger than the average ratio for AGB stars. This excess implies the presence of a cold, extended dust shell, and relatively little hot dust close to the star, as compared to the average mass-losing AGB star. The morphology of detached shells provides a clear indication as to its origin (circular rings due to interrupted mass-loss, paraboloidal bow-shock shaoes due to interaction of the AGB wind with the ISM): hence our proposed PACS observations will thus be able to distinguish between detached shells due to interrupted mass-loss and ISM interactions. |
