The chronological sequence of circumstellar masers - Identifying proto-planetary nebulae

Abstract

The trends arising in circumstellar shells due to the presence or absence of various circumstellar masers are studied as a function of the far-infrared colors of the masers. Several of these trends show up most clearly as a dependence of the detection rate for particular masers on the (25-12) micron color, which is correlated with the rate of mass loss. This principle is used to suggest a chronological sequence for the development of a circumstellar envelope which explains most extant observations. The masers exhibited by a shell can then be used to diagnose the current phase. This approach is used to identify OH/IR stars that may be early examples of the protoplanetary nebula phase.