The dependence of protostar formation on the geometry and strength of the initial magnetic field
Monthly Notices of the Royal Astronomical Society
Oxford University Press
© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
We report results from 12 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 or 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.
BTL acknowledges support from an STFC Studentship and Long Term Attachment grant. This work was also supported by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013 Grant Agreement No. 339248). MRB's visit to Monash was funded by an International Collaboration Award from the Australian Research Council (ARC) under the Discovery Project scheme grant DP130102078. This work used the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the National E-Infrastructure. Calculations were also performed on the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter. This work also made use of the NumPy (van der Welt, Colbert & Varoquax 2011) and Matplotlib (Hunter 2007) Python modules. Rendered plots were produced using the splash (Price 2007) visualization program.
This is the final version of the article. Available from Oxford University Press via the DOI in this record.
Vol. 467, Iss. 3, pp. 3324 - 3337