AAS 199th meeting, Washington, DC, January 2002
Session 137. Topics in Stellar Evolution
Display, Thursday, January 10, 2002, 9:20am-4:00pm, Exhibit Hall

[137.02] On Death Statistics of OH/IR Stars

B. M. Lewis (Arecibo Observatory)

There are two evolutionary scenarios for dead OH/IR stars. The first occurs with the final loss of a stellar envelope, whereupon every star eventually becomes a PN, the prototype being IRAS18455+0448. The smooth, exponential decline of its last 1612\,MHz maser was followed for two years until its peak intensity I\rm 12 < 2\, mJy (Lewis, Oppenheimer, & Daubar ApJ 548, L77). The second scenario occurs when mass-loss declines sharply as the He-shell flash luminosity ebbs away. An AGB star subsequently retraces its history en route to its next thermal pulse some 50-80,000\,yr later, when it is briefly resurrected as an OH/IR star again (Wood & Vassiliadis 1992).

Up to now we have only had evidence for the death of one OH/IR star, so direct confirmation of the second scenario depended on discovering more. This is practical as the net duration of 1612 MHz emission from \vert b \vert > 10\deg OH/IR stars is {~}1700\, years (Lewis ApJ 533, 959). Thus one death" should occur in a sample of 170 stars every ten years, if there is only one emission phase. We have therefore reobserved the 340 stars in the Arecibo sky, with I\rm 12\,>\,100\,mJy when first detected, to develop statistics from a 12 year interval: 4 are now undetectable, while a fifth, IRAS15060+0947, is in terminal decline and is the prototype for the second scenario.

The position of our dead stars on first epoch plots of I\rm 12\, v\,S(25), or their ratio versus IR color, are entirely normal. However all of the new deaths are from stars with V\rm e\,<\,10\, km/s and blue IR colors. When these criteria delimit the sample further, we have 5 deaths from 101 stars, for an emission life t\rm e\,{~}\,240\, yr. Moreover two of the deaths are from the 34 remaining stars with\vert b \vert\,> \,10\deg, which implies at\rm e\,{~}\,200\, yr. These statistics show that their 1612\,MHz emission phase is a recurrent phase that is directly linked to the luminosity changes following after a He- shell flash.