Pumping of Circumstellar OH Main Line Masers

Astronomy and Astrophysics – Astronomy

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Scientific paper

Models are presented for the pumping of the main line OH masers in the circumstellar shells of optically thick OH/IR stars. The effects of non-local FIR line overlaps, collisional excitations, and dust FIR pumping are included in the calculations of the level populations of OH. We also investigate the effect of a possible NIR overlap. These calculations are carried out within the context of a simple model of the outflow and thus incorporate the variation of pumping conditions with radius. The resulting population distribution gives the spatial location and relative strengths of the four ground state maser lines of OH. We show that the non-local nature of the overlaps has a significant effect on the inversions and consequently the location of each maser line. A comparison is made to recent observations of main line emission from OH/IR stars. We find general qualitative agreement between the models and these observations. Though for thick shells FIR overlaps alone appear to be able to reproduce the observations, a NIR overlap may be necessary to explain the observed emission from thinner shells, as has been suggested previously for Type I Miras (Collison & Nedoluha 1993, ApJ, submitted). This research was supported at the Naval Research Laboratory by awards from the National Research Council.

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