AN OH SATELLITE LINE MASER SURVEY OF COOL IRAS SOURCES AND CIRCUMSTELLAR ENVELOPE EVOLUTION

We present the results of a search for 1612 MHz OH circumstellar masers in a colour limited sample of cool IRAS sources. 41 previously unknown detections are reported. This work is the fourth part of the Nancay surveys of IRAS evolved stars. A comprehensive comparison between the results herein and those obtained previously, is developed. Special attention is given to the relationship between the satellite and main line data and, to the evolution of the circumstellar environment along the sequence of reddening colours. The maser profiles indicate an evolving envelope in which the main and satellite line masers are produced in regions with a similar velocity field. Indications for the detachment of a part of the cirumstellar envelope, or its splitting into two distinct envelopes where a peripheral deceleration might occur in the exterior parts of the outer envelope, are deduced from the profiles of the objects exhibiting both a main and satellite line maser, and further suggested by the detection rates. It is shown that, contrary to the expected, the envelope does not homogenize with reddening colours. With a roughly equal contribution, the large number of 35 and 53 mum photons can easily invert the satellite line. Enough IR photons are available to simply pump the main line in many of the objects but for several others, an enhancing mechanism such as line overlap is required. The clumpiness of the circumstellar envelope for a large set of sources is measured. The results obtained have been used to clarify some of the points of the various evolutionary scenarios that have been proposed.