Abstract
Abstract We study the 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, ApJ, 725, 466) are generated by the internal turbulent gas motion before the collision, if the uniform magnetic field strength is 4.0 μG. The collision speed of 10 km s−1 is adopted, which is much larger than the sound speeds and the Alfvén speeds of the clouds. We identify gas clumps with gas densities greater than 5 × 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 μG) models than the weak magnetic field (0.1 μG) 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 the 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 in the isolated, non-colliding cloud models, which are simulated for comparison. We discuss the implications of our numerical results on massive star formation.
Highlights
Massive stars have fundamental influence over the interstellar medium and galactic evolution
We show the position of cores with masses greater than 10 M in the strong B0 models, the Xstrong, Ystrong, and XYstrong models, and the weak B0 model, the Yweak model, overlaid on the plot of column density looking from the collision axis (x-axis)
We show the concave structures of the shocked layer created by the small cloud penetration into the large cloud in the strong B0 models at t = 2.0 Myr, as well as for the Yweak model for comparison
Summary
Massive stars have fundamental influence over the interstellar medium and galactic evolution. Hydrodynamic simulations of CCCs have been carried out to study the formation of dense cores, assuming turbulence in the clouds (Takahira et al 2014; Takahira et al 2018; Shima et al 2018) They discuss that dense cores formed in the shocked layer can increase their mass by accretion of dense gas if turbulence and magnetic fields support the dense cores. Chen & Ostriker (2014) and Chen & Ostriker (2015) have shown that the magnetic fields do not affect the typical mass of pre-stellar cores formed in the colliding clumps in a large molecular cloud They found the formation of low-mass stars in their simulations. Wu et al (2017a) and Wu et al (2017b) have shown that the magnetic fields do not affect star formation in the CCC in their numerical simulations with more typical collision speed of CCCs, and they investigated observational signatures and physical properties of dense regions in the colliding magnetized clouds.
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