A search for water and mainline masers in OH/IR star mimics with strong silicate emission features

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Abstract
IRAS sources with a strong 9.7-μm silicate emission feature have a low-resolution spectral type of 28 or 29, and appear to be associated with oxygen-rich circumstellar shells. Yet ≈ 40 per cent of those searched at Arecibo for 1612-MHz emission have none, despite red IR colours, (25–12) μm>–0.55, that point to a thick shell. We report on a water and mainline OH maser search of a complete sample of these OH/IR star colour mimics, where we find seven new water and five new mainline masers. In aggregate, just 10/60 (17 per cent) of this sample are without any kind of detected maser. The frequency of water and mainline masers in these mimics is about half that in OH/IR stars with the same colour, a result that independently confirms that the shells of mimics are in some way different from those of OH/IR stars. Combining these results with other work on the incidence of masers in O-rich shells, we suggest that the percentage of maserless shells increases from ≈ 11 to ≈ 25 per cent at (25–12) μm = −0.5.

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