Magnetic fields in star formation

This research project  aims at simulating stars and stellar clusters as well as modelling the interstellar medium. Especially interesting is the role magnetic fields play in molecular clouds prior to as well as during the gravitational collapse.

Although the influence of magnetic fields is regarded as vital in the star formation process, only a few magnetohydrodynamics (MHD) simulations have been performed on this subject within the smoothed particle hydrodynamics (SPH) method [d1,d2]. This is largely due to the unsatisfactory treatment of non-vanishing divergence of the magnetic field. Recently smoothed particle magnetohydrodynamics (SPMHD) simulations based on Euler potentials have proven to be successful in treating MHD collapse and fragmentation problems, however these methods are known to have some intrinsical difficulties [d4]. We have performed SPMHD simulations based on a traditional approach evolving the magnetic field itself using the induction equation [d3]. To account for the numerical divergence, we have chosen an approach that subtracts the effects of numerical divergence from the force equation, and additionally we employ artificial magnetic dissipation as a regularization scheme. We apply this realization of SPMHD to a widely known setup, a variation of the 'Boss & Bodenheimer standard isothermal test case', to study the impact of the magnetic fields on collapse and fragmentation. In our simulations [d5], we concentrate on setups, where the initial magnetic field is parallel to the rotation axis. We examine different field strengths and compare our results to other findings reported in the literature. We are able to confirm specific results found elsewhere, namely the delayed onset of star formation for strong fields, accompanied by the tendency to form only single stars. We also find, contrary to other authors, a speed-up of the onset of star formation at intermediate field strengths, and the formation of triple systems. The latter is attributed to magnetic tension forces that aid fragmentation in these cases.

The knowledge concerning the numerical treatment of hydrodynamics and magnetism by SPHMHD at a later stage of the project shall be transferred to the investigation of hydrodynamics effects in (para-) magnetic colloidal systems (see also Phase transitions with hydrodynamics).

The videos are also available in higher quality as .avi:

• strong blast
  
• field loop advection
  
• fast rotor
  
• Orszag-Tang Vortex
• binary star formation
• Boss & Bodenheimer 'standard isothermal collapse test'

b) Old publications

c) Completed work

1. A. Geng, Staatsexamensarbeit: Numerische Analysen zur Sternstrukturberechnung (2008)
2. W. Trick, Bachelor-Arbeit: Numerische Simulationen zur Sternentwicklung im Post-Haupreihenstadium (2010)
3. F. Bürzle, Doktorarbeit: Numerical Studies in Star Formation using Smoothed Particle Magnetohydrodynamics (2012)

d) References

1. J.J. Monaghan, Rep. Prog. Phys. 68, 1703 (2005).
2. V. Springel, MNRAS 364, 1105 (2005).
3. K. Dolag, F. Stasyszyn, MNRAS 398, 1678 (2009).
4. D. Price, M. Bate, MNRAS 377, 77 (2007).
5. F. Bürzle et al., MNRAS 412, 171 (2011).

e) External links (programs, potentials ...)

f) Books