Oh masers as diagnostics of physical conditions in star-forming regions .1. Thermal effects

We have studied the pumping of OH masers in star-forming regions by introducing four pumping mechanisms one at a time. The four mechanisms are collisions, local line overlap, nonlocal line overlap, and external infrared radiation field. Here we restrict ourselves to collisions and local line overlap. This means that we consider relatively small velocity gradients and no significant external infrared radiation. We have used the most accurate collision rates of OH with ortho- and para-H-2 which became available recently and have taken the effects of line overlap into account as accurately as possible using a recently developed formalism. We have found that it makes a qualitative difference whether the OH molecules collide with para-H-2 or ortho-H-2. Thus, maser ovservations may reveal the fractional abundance of ortho-H-2 in star-forming regions. Our calculations have revealed a number of diagnostics of the physical conditions in star-forming regions. These diagnostics are enumerated in our conclusions.