  Recent observations of molecular clouds around rich massive star clusters
including NGC3603, Westerlund 2, and M20 revealed that the formation of massive
stars could be triggered by a cloud-cloud collision. By using
three-dimensional, isothermal, magnetohydrodynamics simulations with the effect
of self-gravity, we demonstrate that massive, gravitationally unstable,
molecular cloud cores are formed behind the strong shock waves induced by the
cloud-cloud collision. We find that the massive molecular cloud cores have
large effective Jeans mass owing to the enhancement of the magnetic field
strength by shock compression and turbulence in the compressed layer. Our
results predict that massive molecular cloud cores formed by the cloud-cloud
collision are filamentary and threaded by magnetic fields perpendicular to the
filament.
