Comparing simulations of ionisation triggered star formation and
  observations in RCW 120
  Massive clumps within the swept-up shells of bubbles, like that surrounding
the galactic HII region RCW 120, have been interpreted in terms of the Collect
and Collapse (C&C) mechanism for triggered star formation. The cold, dusty
clumps surrounding RCW 120 are arranged in an almost spherical shell and
harbour many young stellar objects. By performing high-resolution,
three-dimensional SPH simulations of HII regions expanding into fractal
molecular clouds, we investigate whether the formation of massive clumps in
dense, swept-up shells necessarily requires the C&C mechanism. In a second
step, we use RADMC-3D to compute the synthetic dust continuum emission from our
simulations, in order to compare them with observations of RCW 120 made with
APEX-LABOCA at 870 micron. We show that a distribution of clumps similar to the
one seen in RCW 120 can readily be explained by a non-uniform initial molecular
cloud structure. Hence, a shell-like configuration of massive clumps does not
imply that the C&C mechanism is at work. Rather, we find a hybrid form of
triggering, which combines elements of C&C and Radiatively Driven Implosion
(RDI). In addition, we investigate the reliability of deriving clump masses
from their 870 micron emission. We find that for clumps with more than 100
M_sun the observational estimates are accurate to within a factor of two and
that, even at these long wavelengths, it is important to account for the
radiative heating from triggered, embedded protostars.
