  Investigations into the substructure of massive star forming regions are
essential for understanding the observed relationships between core mass
distributions and mass distributions in stellar clusters, differentiating
between proposed mechanisms of massive star formation. We study the
substructure in the two largest fragments (i.e. cores) MM1 and MM2, in the
infrared dark cloud complex SDC13. As MM1 appears to be in a later stage of
evolution than MM2, comparing their substructure provides an insight in to the
early evolution of massive clumps. We report the results of high resolution SMA
dust continuum observations towards MM1 and MM2. Combining these data with
Herschel observations, we carry out RADMC-3D radiative transfer modelling to
characterise the observed substructure.
  SMA continuum data indicates 4 sub-fragments in the SDC13 region. The nature
of the second brightest sub-fragment (B) is uncertain as it does not appear as
prominent at the lower MAMBO resolution or at radio wavelengths. Statistical
analysis indicates that it is unlikely to be a background source, an AGB star,
or the free-free emission of a HII region. It is plausible that B is a runaway
object ejected from MM1. MM1, which is actively forming stars, consists of two
sub-fragments A and C. This is confirmed by 70 micron Herschel data. While MM1
and MM2 appear quite similar in previous low resolution observations, at high
resolution, the sub-fragment at the centre of MM2 (D) is much fainter than
sub-fragment at the centre of MM1 (A). RADMC-3D models of MM1 and MM2 are able
to reproduce these results, modelling MM2 with a steeper density profile and
higher mass than is required for MM1. The relatively steep density profile of
MM2 depends on a significant temperature decrease in its centre, justified by
the lack of star formation in MM2. A final stellar population for MM1 was
extrapolated, indicating a ...
