Massive core/star formation triggered by cloud-cloud collision: Effect
  of magnetic field
  We study effect of magnetic field on massive dense core formation in
colliding unequal molecular clouds by performing magnetohydrodynamic
simulations with sub-parsec resolution (0.015 pc) that can resolve the
molecular cores. Initial clouds with the typical gas density of the molecular
clouds are immersed in various uniform magnetic fields. The turbulent magnetic
fields in the clouds consistent with the observation by Crutcher et al. (2010)
are generated by the internal turbulent gas motion before the collision, if the
uniform magnetic field strength is 4.0 $\mu$G. The collision speed of 10 km
s$^{-1}$ is adopted, which is much larger than the sound speeds and the
Alfv\'{e}n speeds of the clouds. We identify gas clumps with gas densities
greater than 5 $\times$ 10$^{-20}$ g cm$^{-3}$ as the dense cores and trace
them throughout the simulations to investigate their mass evolution and
gravitational boundness. We show that a greater number of massive,
gravitationally bound cores are formed in the strong magnetic field (4.0
$\mu$G) models than the weak magnetic field (0.1 $\mu$G) models. This is partly
because the strong magnetic field suppresses the spatial shifts of the shocked
layer that should be caused by the nonlinear thin shell instability. The
spatial shifts promote formation of low-mass dense cores in the weak magnetic
field models. The strong magnetic fields also support low-mass dense cores
against gravitational collapse. We show that the numbers of massive,
gravitationally bound cores formed in the strong magnetic field models are much
larger than the isolated, non-colliding cloud models, which are simulated for
comparison. We discuss the implications of our numerical results on massive
star formation.
