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Giant molecular clouds contain supersonic turbulence that can locally heatsmall fractions of gas to over 100 K. We run shock models for low-velocity,C-type shocks propagating into gas with densities between 10^3 and 10^5 cm^(-3)and find that CO lines are the most important cooling lines. Comparison tophotodissociation region (PDR) models indicates that mid-J CO lines (J = 8-7and higher) should be dominated by emission from shocked gas. In Papers I andII we presented CO J = 3-2, 8-7, and 9-8 observations toward four primarilyquiescent clumps within infrared dark clouds. Here we fit PDR models to thecombined spectral line energy distributions and show that the PDR models thatbest fit the low-J CO emission underpredict the mid-J CO emission by orders ofmagnitude, strongly hinting at a hot gas component within these clumps. Thelow-J CO data clearly show that the integrated intensities of both the CO J =8-7 and 9-8 lines are anomalously high, such that the line ratio can be used tocharacterize the hot gas component. Shock models are reasonably consistent withthe observed mid-J CO emission, with models with densities near 10^(4.5)cm^(-3) providing the best agreement. Where this mid-J CO is detected, the meanvolume filling factor of the hot gas is 0.1%. Much of the observed mid-J COemission, however, is also associated with known protostars and may be due toprotostellar feedback.

Mid-J CO Shock Tracing Observations of Infrared Dark Clouds III: SLED fitting