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B. M. Lewis (Arecibo Observatory)
The immediate precursors of O-rich proto planetary nebulae (PPN) are usually OH/IR stars. Indeed the prototype PPN, IRAS\,18095+2704, still has OH masers. Since PPN are actively evolving into planetary nebulae, which characteristically have ionised rather than neutral gas shells, the decay and loss of circumstellar masers is expected to occur during the early PPN phase of shell evolution. Thus the exponential decline of the last 1612\,MHz maser of IRAS\,18455+0448 over 1998- 2000 points to its probable PPN status, and heralds its imminent death as an OH/IR star. The 1612\,MHz masers in this star had peak intensities of 2.09 & 0.64\,Jy at first detection in 1988, where the intensity of the stronger maser at the same resolution is now ~7\,mJy. On the other hand the ratio of the number of PPN with \vert b \vert > 10\circ in the Arecibo sky to OH/IR stars and mimics (IRAS sources with the IR colors of OH/IR stars without 1612\,MHz masers) shows that the immediate precursors of OH/IR stars are mimics. The first emergence of 1612\,MHz masers from a mimic's shell then flags its birth as an OH/IR star. Prototype examples of recent births are IRAS\, 19566+3423 and V1511\,Cyg. The ~1\,Jy 1612\,MHz masers of 19566+3423 at first discovery in 1986 increased linearly over the next 6\,yr. But by 1999 they had transformed themselves to extend over more than twice the original velocity range in both 1665 and 1612\,MHz with intensities ~3\,Jy. Clearly some OH masers rise from nothing in less than 8\,yr. Those in V1511\,Cyg now have the usual, classic, twin-peaked structure of an OH/IR star, with intensities at discovery in May 1999 of 72 and 332 mJy. But four earlier published searches for masers over 1988-1991 had detected no masers.
The author(s) of this abstract have provided an email address for comments about the abstract: blewis@naic.edu