| 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. |
