Dust radiation fields and pumping of excited state OH masers in W3(OH)

I show that it is quite possible to obtain saturating amplification in the 13.44-GHz maser which is so far unique to W3(OH), using a combined radiativ e and collisional pumping scheme. The dominant radiative part of the pump i nvolves far-infrared line overlap, and the far-infrared continuum is provid ed by dust, modelled as either a two-component mixture or composite grains. Transport of the far-infrared radiation is carried out via the accelerated lambda iteration method. The observational link between 13.44-GHz and 6035 -MHz masers is reproduced by the model. Inadequate amplification in Sobolev models probably results from the optical depth limitations imposed by this approximation. I review the dust models used in far-infrared pumping model s of OH masers, and conclude that the main consequence of moving from skewe d blackbody functions to more sophisticated models is a selective pumping e nhancement when ice mantles are included.