Emission and cooling by CO in interstellar shock waves

We have calculated emission by CO molecules from interstellar shock waves. Two approximations have been used to determine the population densities, nJ, of the rotational levels, J: steady state (∂/∂t≡ 0) and statistical equilibrium (d/dt≡ 0). A large-velocity-gradient approximation to the line-transfer problem was adopted in both cases. We find that there can be substantial differences between the values of the integrated rotational line intensities calculated in steady state and in the limit of statistical equilibrium. On the other hand, although CO can be the dominant coolant towards the rear of the cooling flow which follows the dynamical heating of the gas, the rate of cooling computed assuming statistical equilibrium is likely to be reasonably accurate, given that the limit of statistical equilibrium is approached in this region.