Mid-J CO Shock Tracing Observations of Infrared Dark Clouds. III. SLED Fitting

Journal
Date Issued
2016
Author(s)
Pon, A.
•
Kaufman, M. J.
•
Johnstone, D.
•
Caselli, P.
•
•
Butler, M. J.
•
Jiménez-Serra, I.
•
Palau, A.
•
Tan, J. C.
DOI
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 103 and 105 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 \to 7 and higher) should be dominated by emission from shocked gas. In Papers I and II we presented CO J = 3 \to 2, 8 \to 7, and 9 \to 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 \to 7 and 9 \to 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.
Volume
827
Issue
2
Start page
107
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2016ApJ...827..107P
Rights
open.access
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