LINESHAPES OF INTENSE OH MASERS IN STAR-FORMING REGIONS

Using an extensive model of OH masers in star-forming regions, we consider the importance of redistribution of the velocity of the masing molecules, calculating the form of the evolving maser lineshape on two limiting assumptions of no velocity redistribution (NVR) and of complete velocity redistribution (CVR). We show that maser emission lines narrow and rebroaden for NVR, as masers saturate, but remain narrow for CVR, in agreement with earlier work of Goldreich and Kwan. For NVR, competitive gain leads certain lines to form narrow double peaks. CVR causes the maximum gain to increase by a factor typically of similar to 3. We consider the effects of velocity fields in the direction of maser propagation and find the surprising general result that maser amplification is substantially greater for propagation along a non-zero velocity gradient for CVR.