This paper presents results from modelling 450 μm and 850 μm continuum and +HCO line observations of three distinct cores of an infrared dark cloud (IRDC) directed toward the W51 GMC. In the sub-mm continuum these cores appear as bright, isolated emission features. One of them coincides with the peak of 8.3 μm extinction as measured by the Midcourse Space Experiment satellite. Detailed radiative transfer codes are applied to constrain the cores’ physical conditions to address the key question: Do these IRDC-cores harbour luminous sources? The results of the continuum model, expressed in the χ2 quality-of-fit parameter, are also constrained by the absence of 100 μm emission from IRAS. For the sub-mm emission peaks this shows that sources of ∼ 300 {L}⊙ are embedded within the cores. For the extinction peak, the combination of continuum and +HCO line modelling indicates that a heating source is present as well. Furthermore, the line model provides constraints on the clumpiness of the medium. All three cores have similar masses of about 70{-}150 {M}⊙ and similar density structures. The extinction peak differs from the other two cores by hosting a much weaker heating source, and the sub-mm emission core at the edge of the IRDC deviates from the other cores by a higher internal clumpiness.
