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arXiv · 1701.08741

The dependence of protostar formation on the geometry and strength of the initial magnetic field

Benjamin T. Lewis·Matthew R. Bate
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Abstract

We report results from twelve simulations of the collapse of a molecular cloud core to form one or more protostars, comprising three field strengths (mass-to-flux ratios, μ, of 5, 10, and 20) and four field geometries (with values of the angle between the field and rotation axes, θ, of 0°, 20°, 45°, and 90°), using a smoothed particle magnetohydrodynamics method. We find that the values of both parameters have a strong effect on the resultant protostellar system and outflows. This ranges from the formation of binary systems when μ = 20 to strikingly differing outflow structures for differing values of θ, in particular highly suppressed outflows when θ = 90°. Misaligned magnetic fields can also produce warped pseudo-discs where the outer regions align perpendicular to the magnetic field but the innermost region re-orientates to be perpendicular to the rotation axis. We follow the collapse to sizes comparable to those of first cores and find that none of the outflow speeds exceed 8 km s$^{-1}$. These results may place constraints on both observed protostellar outflows, and also on which molecular cloud cores may eventually form either single stars and binaries: a sufficiently weak magnetic field may allow for disc fragmentation, whilst conversely the greater angular momentum transport of a strong field may inhibit disc fragmentation.

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Benjamin T. Lewis, Matthew R. Bate. 2017-01-30. The dependence of protostar formation on the geometry and strength of the initial magnetic field. https://doi.org/10.1093/mnras%2Fstx271

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