We present radiation-magnetohydrodynamic simulations aimed at studying evolutionary properties of H II regions in turbulent , magnetised , and collapsing molecular clouds formed by converging flows in the warm neutral medium .
We focus on the structure , dynamics and expansion laws of these regions .
Once a massive star forms in our highly structured clouds , its ionising radiation eventually stops the accretion ( through filaments ) toward the massive star-forming regions .
The new over-pressured H II regions push away the dense gas , thus disrupting the more massive collapse centres .
Also , because of the complex density structure in the cloud , the H II regions expand in a hybrid manner : they virtually do not expand toward the densest regions ( cores ) , while they expand according to the classical analytical result towards the rest of the cloud , and in an accelerated way , as a blister region , towards the diffuse medium .
Thus , the ionised regions grow anisotropically , and the ionising stars generally appear off-centre of the regions .
Finally , we find that the hypotheses assumed in standard H II-region expansion models ( fully embedded region , blister-type , or expansion in a density gradient ) apply simultaneously in different parts of our simulated H II regions , producing a net expansion law ( R \propto t ^ { \alpha } , with \alpha in the range of 0.93-1.47 and a mean value of 1.2 \pm 0.17 ) that differs from any of those of the standard models .