2001ApJ...560..400L

High-latitude, |b|≥10°, asymptotic (AGB) stars in their superwind phase have distinctive IR colors, which make them easy to identify and count. But the expansion velocity (V_e) versus color plots of these stars are a puzzle, with features implying (1) a step increase in dM/dt and (2) short-term changes in the circumstellar shell that are enhanced by increased metallicity. The net duration of their superwind phase is ∼3700 yr when estimated from the relative frequency of associated proto-planetary nebulae and the ∼103 yr expansion age of one, IRAS 18095+2704. Since these stars lose on average ∼0.04 M_☉ during the superwind phase and have progenitor masses circa 1 M_☉, they must lose more than 0.3 M_☉ prior to the AGB.

A single 3700 yr superwind episode, however, cannot explain the 2-18 km.s^–1 range of expansion velocities exhibited by these stars. This range is best understood as resulting from a cyclical superwind, wherein stars lose most of their envelope mass in three to four discrete episodes of ever larger V_e, with each episode being triggered by a sudden onset in the coupling of photon momentum by dust to gas near a thermal pulse. The resulting newly accelerated shell thereafter quickly expands beyond the protective dust shroud of the prior shell, which allows interstellar UV to degrade its molecules. This turns the shell into an OH/IR star color mimic (a shell with the dM/dt of an OH/IR star without 1612 MHz masers), until enough dust is again in place beyond the shell to allow molecules the longevity needed to support masers. Two by-products of this model are the natural explanation it offers for why V_e(CO) >V_e(OH), as well as for the occurrence and integration of mimics into the normal development of a shell. The circumstellar shells of high-latitude OH/IR stars are thus cyclically transient phenomena.