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

TL;DR

This study investigates mid-J CO line emissions in infrared dark clouds, demonstrating that shock models better explain the hot gas component than PDR models, with implications for understanding turbulence and feedback in molecular clouds.

Contribution

The paper introduces shock models for low-velocity C-type shocks to explain mid-J CO emissions, highlighting the hot gas component in IRDCs beyond PDR predictions.

Findings

Mid-J CO lines are dominated by shock-heated gas.

Shock models with densities near 10^4.5 cm^(-3) fit observations well.

Hot gas filling factor is approximately 0.1% in observed regions.

Abstract

Giant molecular clouds contain supersonic turbulence that can locally heat small 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 to photodissociation region (PDR) models indicates that mid-J CO lines (J = 8-7 and higher) should be dominated by emission from shocked gas. In Papers I and II we presented CO J = 3-2, 8-7, and 9-8 observations toward four primarily quiescent clumps within infrared dark clouds. Here we fit PDR models to the combined spectral line energy distributions and show that the PDR models that best fit the low-J CO emission underpredict the mid-J CO emission by orders of magnitude, strongly hinting at a hot gas component within these clumps. The low-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 to characterize the hot gas component. Shock models are reasonably consistent with the 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 mean volume filling factor of the hot gas is 0.1%. Much of the observed mid-J CO emission, however, is also associated with known protostars and may be due to protostellar feedback.