Do expanding shells require special conditions in molecular clouds?
Anthony Whitworth, Thomas Bisbas, David Hubber, Richard Wuensch
Shells, bubbles, and triggered star formation around young massive stars are frequently observed at all common wavelengths and throughout the galaxy. In numerical simulations with the smoothed particle hydrodynamics code Seren, we investigate the formation of shell-like structures, which are swept up by expanding HII regions around young massive stars. We treat the ionising radiation with a well-tested HEALPix-based algorithm. The initial molecular cloud surrounding the ionising source is assumed to be fractal on large scales, where the fractal dimension Df is variable. We allow Df to span a broad range, covering the observed values (Df=2.3−2.7), as well as even more uniform setups (up to Df=2.9). This model allows us to fully control the clumpiness and the richness in molecular cloud structure, which is impossible with conventional turbulent molecular cloud setups.
We find that the formation of a partially intact shell structure requires a rather uniform molecular cloud with a fractal dimension of Df > 2.7, which implies that the interstellar gas in the vicinity of a young massive star is distributed rather evenly as compared to typically observed molecular clouds. In my talk I will discuss fractal molecular clouds, show the effect of ionising radiation for different initial conditions, and discuss possible conclusions on the formation of massive stars.