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

Abstract

The evolution, pulsation, and nature of the type II OH/IR stars is analyzed using the extensive data base and trends described in the third paper in this series. Using evolutionary tracks and pulsation theory, the lowest luminosity (M/sub bol/>-5) masers on the asymptotic giant branch (AGB) are shown to be cool, fundamental mode pulsators which are possibly metal rich. Comparison of the positions in the period-luminosity plane of the masers width the long-period variables in the Magellanic Clouds and in the Galactic bulge suggests that in fact all of the redder (>5) and more efficient (>1) masers on the AGB are fundamental mode pulsator. The periods of several of the bluer (<3) and less efficient (<0) AGB masers are consistent with first overtone pulsation; these objects are similar to the classical IRC Miras except bolometrically brighter. As well as the AGB maser sources, there is a class of sources which are core helium burning supergiants, but whether they pulsate in the fundamental or first overtone models or not at all is not clear at present.