Statistics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001aas...19913702l&link_type=abstract
American Astronomical Society, 199th AAS Meeting, #137.02; Bulletin of the American Astronomical Society, Vol. 33, p.1512
Statistics
Scientific paper
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 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 ěrt b ěrt > 10{° } 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 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 12 v S(25), or their ratio versus IR color, are entirely normal. However all of the new deaths are from stars with V 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 e { ~ } 240 yr. Moreover two of the deaths are from the 34 remaining stars withěrt b ěrt > 10{° }, which implies at 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.
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