Line profiles in masers: Velocity relaxation
Populations and velocity profiles of the rotational levels of the water molecule are calculated under conditions typical of those in astrophysical water masers. Trapped infrared radiation as well as elastic and inelastic collisions are included. The distortion of velocity profiles by a strong, narrow maser line is found with minimal assumptions and approximations. This provides the first detailed determination of the rate of velocity relaxation, accurate values of which are essential for interpreting maser line profiles in general and for inferring maser luminosities in particular. Relaxation rates for the 22-GHz maser at densities of 109 cm−3 are 2.1 s−1 and 5.3 s−1 for temperatures of 400 K and 1000 K, respectively, including reasonable elastic collision rates. Under these and other conditions, as well as for other transitions, the relaxation rate far exceeds the loss rate-usually by more than a factor of 10. Therefore, line narrowing can continue to an intensity that is at least 10 times the saturation intensity.
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