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A. Burkert (Max-Planck-Institut for Astronomy, Heidelberg, Germany), D. Balsara (NCSA/UIUC)
The study of protostellar core collapse and fragmentation is one of the most interesting and fascinating problems in astrophysics. It is also very challenging owing to the fact that the mesh-based Jeans number has to be properly maintained in the course of the computation. The multiple length scales that need to be represented inevitably call for AMR simulations. Studies so far have mainly been hydrodynamical, see Truelove et al (1997) and Boss et al (2000). AMR-MHD simulations have so far been impossible because of the problem to carry out divergence-free calculations on AMR meshes. In a breakthrough paper Balsara (2001) showed that divergence-free AMR-MHD calculations are indeed possible. We have used the resulting RIEMANN framework for AMR-MHD for studying protostellar core collapse and fragmentation.
We have studied two sets of initial conditions: a gravitationally unstable, rotating cloud core with a 50 density perturbation, where the magnetic field is aligned or orthogonal to the rotation axis. Without magnetic fields this initial condition has been shown to form a wide binary system. We find that the orthogonal rotator loses angular momentum very fast, as anticipated already by Mouschovias and Paleologou (1985) preventing the formation of a binary. The aligned rotator loses angular momentum much slower but again, no binary forms. This case develops a very interesting phenomenology which results from the competition between the infall time, the magnetic breaking time and the ambipolar diffusion time and their interplay as the system evolves. The aligned rotator also self-consistently develops a magneto-centrifugally driven outflow as expected theoretically.
The adopted initial conditions were ideal for fragmentation and binary formation. Yet, the magnetic field still could efficiently remove angular momentum and prevent binary formation. This result is puzzling given the fact that most stars tend to form in binaries. It might indicate that magnetic fields cannot play an important role during the protostellar collapse phase.
The author(s) of this abstract have provided an email address for comments about the abstract: burkert@mpia-hd.mpg.de