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Session 48 - Circumstellar Envelopes.
Display session, Tuesday, January 14
Metropolitan Ballroom,
A complete sample of color selected IRAS sources with S(25)\ \ge2\,Jy in the Arecibo sky (0^o\,\le\,\delta\,\le \,37^o) was examined previously for 1612\,MHz masers. The resulting set of 420 OH/IR stars has now been searched for mainline OH masers at Arecibo (ApJ Suppl., April 1997) and for 22\,GHz water masers at Effelsberg (Aamp;AS 116, 117). With our high deployed sensitivity we have an overall detection rate of 59.6% for the mainlines and 55.4% for water, though the detectability of both masers is sensitive to the IR flux intensity when S(25)\,<\,20\,Jy. Nevertheless, the bluest objects exhibit higher (\approx80%) detection rates, while there is a decrease in detections from redder shells: this decrease begins circa (25-12)\,\mum = -0.55 for mainline masers, and abruptly circa (25-12)\,\mu m = -0.35 for water. These trends confirm the descriptive sequence of the chronological scenario for masers in circumstellar shells, though some proto planetary nebulae and very red OH/IR stars have water masers and mainline masers are detected in shells with (25-12)\,\mum\,\ge\,-0.2. There is no very marked tendency for red OH/IR stars with water masers to also have mainline masers, and vice-versa, though there is a hint that the bluest objects without water tend to be without mainline masers too. Estimates for the expansion velocities of many OH/IR stars with just a single 1612 MHz feature are obtained by taking account of their water and/or mainline data. Mainline emission from OH/IR stars is generally weaker than 1612 MHz emission, and it is very rare for both masers to have their strongest features at exactly the same velocity. The addition of both 1665 amp; 1667\,MHz data for BC Tau = 05506+2414 showing the same single broad feature at exactly the same velocity in each line suggests that this is a pre main sequence object rather than a proto planetary nebula.
