| IRAS sources with a strong 9.7-mum 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) mu>-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) mum=-0.5. |
Comments:
Spectral type was set to:
""S"", where only one peak was found.
""D"", where two peaks were found.
The columns for the blue- and red-shifted velocities were filled with the two highest peaks listed. The leftmost and the rightmost peaks were used to calculate the radial velocity vrad and the expansion velocity vexp of the shell.
Velocity resolution: 0.44 km/s
Upper limit for non-detections: 0.030 Jy (3σ)
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