CIRCUMSTELLAR MASERS AROUND OXYGEN-RICH STARS

The far-IR colors of bright oxygen-rich stars define a clear sequence from sources along the blackbody curve with temperatures > 2000 K, across a small "gap" at color temperatures in the range 1000-2000 K, down to temperatures of ∼ 300 K, where stars are heavily obscured. Circumstellar masers follow the same sequence from the red edge of the gap through the whole temperature sweep, which, with the addition of the heavily obscured OH/IR stars, extends even further to the red. This survey of the detection statistics exhibited by masers in circumstellar shells in association with their far-IR colors allows the evolution of shells to be traced from their inception, when stellar types are generally of low M type, through the steady addition of new masing species as a shell increases its ability to shield molecules from interstellar UV photodissociation, while it grows in radius and density. This has been previously foreshadowed as part of the Chronological Paradigm (Lewis 1989a).