Rapid and efficient mass collection by a supersonic cloud-cloud
  collision as a major mechanism of high-mass star formation
  A supersonic cloud-cloud collision produces a shock-compressed layer which
leads to formation of high-mass stars via gravitational instability. We carried
out a detailed analysis of the layer by using the numerical simulations of
magneto-hydrodynamics which deal with colliding molecular flows at a relative
velocity of 20 km s$^{-1}$ (Inoue & Fukui 2013). Maximum density in the layer
increases from 1000 cm$^{-3}$ to more than $10^{5}$ cm$^{-3}$ within 0.3 Myrs
by compression, and the turbulence and the magnetic field in the layer are
amplified by a factor of $\sim 5$, increasing the mass accretion rate by two
orders of magnitude to more than $10^{-4}$ $M_{\odot}$ yr$^{-1}$. The layer
becomes highly filamentary due to gas flows along the magnetic field lines, and
dense cores are formed in the filaments. The massive dense cores have size and
mass of 0.03 -- 0.8 pc and 8 -- 50 $M_{\odot}$ and they are usually
gravitationally unstable. The mass function of the dense cores is significantly
top-heavy as compared with the universal IMF, indicating that the cloud-cloud
collision triggers preferentially the formation of O and early B stars. We
argue that the cloud-cloud collision is a versatile mechanism which creates a
variety of stellar clusters from a single O star like RCW120 and M20 to tens of
O stars of a super star cluster like RCW38 and a mini-starburst W43. The core
mass function predicted by the present model is consistent with the massive
dense cores obtained by recent ALMA observations in RCW38 (Torii et al. 2019)
and W43 (Motte et al. 2018) considering the increasing evidence for
collision-triggered high-mass star formation, we argue that cloud-cloud
collision is a major mechanism of high mass star formation.
