Mainline OH detection rates from blue circumstellar shells.

We identified 240 accessible circumstellar shells with water and/or SiO masers, and used the Arecibo and Nancay radio telescopes to search for mainline OH emission from them. Our targets are often Mira variables without previously known mainline masers. This search results in 89 detections, of which 77 are new. The probability of detecting a maser is larger once a water maser is known, and becomes progressively larger the thicker and redder the shell. Nevertheless, almost all of our examples of solitary 1665 MHz masers, rather than the joint occurrence of both 1665 and 1667 MHz masers, are in the bluest shells. The IRAS low resolution spectral type is the strongest factor correlating with the mainline detection rate. We find that 67% of objects with a silicate emission feature exhibit masers, whereas only 27% of objects with a comparatively featureless In type do. These rates are colour insensitive. We ascribe this clearcut difference to differing UV extinction properties of the two grain types, which is likely to result from differing grain-size distributions. The IR colour sensitivity of the overall mainline detection rate is thus almost entirely an incidental artifact of the changing proportion of the two grain types with colour. Inferentially, since 90% of the sample exhibit water masers, and the proportion of blue sources with silicate features is substantially larger than an unbiased selection from the IRAS Point Source Catalog would give, the incidence of water masers is similarly sensitive to spectral type.