On Dead OH/IR Stars

Abstract

Because the net duration of all 1612 MHz emission phases from |b| > 10° OH/IR stars is ~1700 yr, one death, as evidenced by the disappearance of all 1612 MHz masers, should occur in a sample of 170 every 10 yr, on average, if they have only one emission phase. We report here on the reobservation after 12 yr of the 328 OH/IR stars in the Arecibo sky with a peak first-epoch I1612 > 100 mJy. Four of these now have undetectable 1612 MHz masers, while those from a fifth, FV Boo, appear to be in terminal decline. The blue IR colors, 315 day period of FV Boo, and presence of water and SiO masers in several of the newly identified dead OH/IR stars all suggest that these objects are still asymptotic giant branch (AGB) stars rather than proto-planetary nebulae. This conclusion suggests that most oxygen-rich AGB stars pass through the OH/IR star phase more than once. The positions of the dead OH/IR stars on first-epoch plots of I1612 versus S(25) and on plots of the ratio of these quantities versus IR color are entirely normal. However, all of the dead OH/IR stars have small, less than 12 km s-1, expansion velocities, and all but one have blue IR colors. When these criteria are used to further delimit the sample, the five dead OH/IR stars in a sample of 112 imply a mean 1612 MHz emission life te ~ 314 (+387, -97) yr. The short duration of this emission phase is readily understood if oxygen-rich AGB stars toward the end of the AGB pass through a brief OH/IR star phase after a thermal pulse, when, as often happens, they are not bright enough to support heavy mass loss on the luminosity ascent to a thermal pulse. Copious AGB mass loss is thus frequently triggered by events that follow a thermal pulse.