OH/IR masers. IV. Evolution, pulsation and nature of the sources.

The evolution, pulsation, and nature of the type II OH/IR stars is analyzed using the data base described in the third paper in this series. Using evolutionary tracks and pulsation theory, the lowest luminosity (M_bol > -5) masers on the asymptotic giant branch (AGB) are shown to be cool, fundamental mode pulsators which are possibly metal rich. The periods of several of the bluer and less efficient AGB masers are consistent with first overtone pulsation. There is a class of sources which are core helium burning supergiants, but whether they pulsate in the fundamental or first overtone modes or not at all is not clear at present. An evolutionary scenario is developed for the 1 - 9 M_sun; (degenerate core type VM) maser involving evolution up the AGB as a classical Mira pulsating in first overtone with mild IR excess and OH emission, then a sudden switch to fundamental mode. Mass loss increases dramatically as a result of the mode switch creating a thick circumstellar shell, an extreme IR excess, and the star becomes a radio-luminous type II OH/IR star.