Abstract
Eight OH-IR stars have been monitored at the satellite- and main-line frequencies of OH. Problems are addressed relating to the pump mechanisms responsible for the maser inversion, the degree of saturation of the masers, and the linear dimensions of the circumstellar shell. The following conclusions have been reached. The satellite- and main-line masers are both radiatively pumped, almost certainly by infrared photons. The OH lags the optical emission by 10-50 days. The data suggest that the linear dimension of the shell is a few times 10 to the 16th cm, in agreement with theoretical predictions. The satellite-line masers are always saturated, but the main-line masers may or may not be. In addition, the anomalous source U Ori is studied. By combining the present data with those of Cimerman (1979), it is shown that the 1612-MHz maser emission of this source follows the optical emission through individual cycles but at reduced mean intensities for each successive cycle.
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