Comparing simulations of ionization triggered star formation and observations in RCW 120

Massive clumps within the swept-up shells of bubbles, like that surrounding the galactic H II 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 smoothed particle hydrodynamics simulations of H II 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 (The Large APEX BOlometer CAmera) at 870 μm. 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 radiation driven implosion (RDI). In addition, we investigate the reliability of deriving clump masses from their 870 μm emission. We find that for clumps with more than 100 M_⊙ the observational estimates are accurate to within a factor of 2, while the agreement between simulated and observed masses is much worse for smaller clumps. We also find that even at 870 μm it is important to account for the radiative heating from triggered, embedded protostars.