Infrared Limb Brightening in the Barnard 5 Cloud

The authors present IRAS and ¹³CO data for the dark cloud Barnard 5. The 100 μm and ¹³CO emission are well correlated and suggest that the 100 μm emission traces the dust and gas column density through the cloud. The 12 μm emission, on the other hand, is anticorrelated with the overall opacity in the cloud which the authors interpret as limb brightening in the context of a model wherein small dust grains, probably PAHs, are transiently excited by the absorption of UV photons in a thin shell around the denser parts of the cloud.