MAINLINE OH DETECTION RATES FROM BLUE CIRCUMSTELLAR SHELLS

We identified 240 accessible circumstellar shells with water and/or Si O masers, and used the Arecibo and Nancay radio telescopes to search f or mainline OH emission from them. Our targets are often Mira variable s 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 large r the thicker and redder the shell. Nevertheless, almost all of our ex amples of solitary 1665 MHz masers, rather than the joint occurrence o f both 1665 and 1667 MHz masers, are in the bluest shells. The IRAS lo w resolution spectral type is the strongest factor correlating with th e mainline detection rate. We find that 67% of objects with a silicate emission feature exhibit masers, whereas only 27% of objects with a c omparatively featureless In type do. These rates are colour insensitiv e. We ascribe this clearcut difference to differing UV extinction prop erties of the two grain types, which is likely to result from differin g grain-size distributions. The IR colour sensitivity of the overall m ainline detection rate is thus almost entirely an incidental artifact of the changing proportion of the two grain types with colour. Inferen tially, since 90% of the sample exhibit water masers, and the proporti on 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.