  This paper presents results from modelling 450 micron and 850 micron
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 micron 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
$\chi^2$ quality-of-fit parameter, are also constrained by the absence of 100
micron emission from IRAS. For the sub-mm emission peaks this shows that
sources of 300 solar luminosities 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 solar masses 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.
