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McKee, C.F., J.W.V. Storey, D.M. Watson, and S. Green, 1982: Far-infrared rotational emission by carbon monoxide. Astrophys. J., 259, 647-656, doi:10.1086/160200.
Accurate theoretical collisional excitation rates are used to determine the emissivities of CO rotational lines for n(H2) greater than or equal to 104 /cm3, 100 K less than T less than 3000 K, and J less than or equal to 60 under the assumption that the lines are optically thin. An approximate analytic expression for the emissivities which is valid in this region is obtained. Population inversions in the lower rotational levels occur for densities n(H2) approximately equal to 103-105 /cm3 and temperatures greater than or approximately equal to 50 K provided photon trapping is unimportant. Interstellar shocks observed edge-on are a potential source of weak millimeter-wave CO maser emission. The CO rotational cooling function suggested by Hollenbach and McKee is verified, although their numerical value is several times too large at low densities.
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