Published online by Cambridge University Press: 14 August 2015
A small percentage of infrared stars, selected on the basis of their IR brightness at 2.2 µm and their redness (I–K colour index), show 18 cm OH emission, and some exhibit 1.35 cm H2O emission; further investigations identify some of these objects as M-type supergiants and the remainder as M-type giant long-period variable stars.
For the strongest 1612 MHz OH emitters, long-baseline interferometry shows that the emission arises from a large number of small-diameter features distributed over a much larger, approximately elliptical region corresponding to a circumstellar cloud. Models for the geometry of the emitting regions are discussed.
The OH characteristics with respect to the ratios of line intensities for different transitions and the velocity structure are distinctive, not being found in other types of OH source; consequently a large number of new OH emitters can be recognized as probably IR/OH objects despite the absence of IR or optical data; the precise classification of these sources is considered together with their relevance in understanding the identified sources.