On the Transience of Circumstellar Shells about |b| ≥ 10o OH/IR Stars. I. Basic Statistics

Abstract

There are 62 OH/IR stars in the Arecibo sky with |b| ≥ 10°, |l| < 90°, and a 25 μm flux S(25) ≥ 2 Jy. These were identified as a result of a complete OH survey of color-selected IRAS sources, which also identified four O-rich proto-planetary nebulae (PPNs). Since OH/IR stars evolve into PPNs, the ~103 yr expansion age of one, 18095+2704, calibrates an ~1670 yr average net duration for 1612 MHz emission from high-latitude stars. This is comparable with the 770 yr wind travel time from the photosphere out to the 1612 MHz emission zone of WX Psc and the ~500 yr duration of the luminosity spike following after a thermal pulse. There is, however, only one PPN candidate without an OH maser to match to the sample's 118 OH/IR-star color mimics (circumstellar shells with the IR colors of OH/IR stars that do not exhibit 1612 MHz masers), where eight are implied if mimics independently evolve through a PPN phase. Most mimics are therefore likely to be the immediate precursors of OH/IR stars. The average net duration of heavy mass loss in a superwind is then estimated from the ratio between the number of IR sources and PPNs at ~3700 yr. However, the absence of any O-rich PPNs to match to OH/IR stars and mimics in the Galactic anticenter region (AGC), even though there are two carbon stars and four C-rich PPN with appropriate IR colors there, offers a strong hint that most OH/IR stars in the AGC exit their asymptotic giant branch phase as C-rich PPNs. This carries the direct implication that the superwind phase in high-latitude stars is usually a cyclical phase.