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B. M. Lewis (Arecibo Observatory)
There are 69 OH/IR stars with \vert b \vert > 10\deg and a 25 {\mu }m flux S(25) > 2\,Jy in the complete Arecibo 1612\,MHz survey of color-selected IRAS sources. WX Psc is one. There are also \ge4 (likely 6) proto planetary nebulae with these parameters. As OH/IR stars evolve into PPN, the expansion age of one, 18095+2704, at \approx 110\,yr gives their time scale. Thus the net duration of their 1612\,MHz emission phases is (69/6 + 1) * 110\,\approx \,1400\, yr. This is comparable with a wind travel-time ({\Delta }t c/2\, ve) \approx 770\,yr from the photosphere to the 1612\,MHz emitting region, which derives from the phase- lag between the blue and red peaks of WX Psc (van Langevelde et al. A&A 239, 193). It is also comparable to the {~}500\,yr duration of the luminosity spike following on a thermal pulse in the host AGB star, as energy from its He-shell flash is radiated (e.g. Wood & Zarro ApJ 247, 247). Our objects also fall into four clumps on a ve-color plot, which suggests they come from 3-5 thermal-pulse cycles. The emission phase in any one cycle is thus \ll500\,yr. Most are only likely to exhibit masers while the luminosity spike decays.
A similarly transient emission phase for all OH/IR stars is implied by the well-known weakness of CO emission from OH/IR stars in general: steady state models predict rCO\,\approx \,10\,*\,rOH, a state rarely if ever achieved. Likewise the existence of OH/IR star color mimics, objects which on the basis of their IR colors should exhibit 1612\,MHz masers but do not (Lewis ApJ 396, 251), follows as a mandatory result of the abrupt onset of the superwind. The consequent rapid increase in ve causes the shell to outstrip its prior dust distribution, which drastically shortens the longevity of OH molecules in the new expanding shell until adequate dust shielding against interstellar UV is again in place at much larger radii. Thus most mimics are a natural consequence of the onset of a superwind, and their high frequency (>40%) among color- selected sources points to the transient nature of the 1612\, MHz emission phase.
The author(s) of this abstract have provided an email address for comments about the abstract: blewis@naic.edu