CARBON STARS WITH EXCESS EMISSION AT 60-MU-M WAVELENGTH

The Infrared Astronomical Satellite (IRAS) discovered that, in a 12-25-60 mum color-color diagram, many optically identified carbon stars emit excess flux at 60 mum wavelength. It is generally agreed that the excess flux is emitted by a ''detached'' remnant dust shell ejected in a bygone episode of relatively rapid mass loss while the star was on the asymptotic giant branch. However, the chemical composition (C/O greater or less than unity?) and the structure (geometrically thick or thin?) of the shells, and their relevance to derivation of carbon star lifetimes are all controversial so that the place of these stars in the larger scheme of stellar evolution is unclear. Model fits to stars with extended far infrared and CO emission indicate that the detached circumstellar shells are geometrically thin rather than thick. Geometrically thin shells imply dimensions sufficiently small that HCN emission can be excited in the vicinity of the region that emits the excess 60 mum flux and published HCN data indicate that most of the shells have C/O > 1. Then carbon star lifetimes can be reconciled with the 10(5)-10(6) yr estimates based on statistical and theoretical considerations rather than the significantly shorter lifetimes derived by various researchers who assume that the detached envelopes are all oxygen-rich. The underlying cause of the variable mass loss rates remains unclear; repeated helium shell flashes seem to be the most plausible explanation.