**AAS 198th Meeting, June 2001**

*Session 65. Computational Astrophysics*

Display, Wednesday, June 6, 2001, 10:00am-7:00pm, Exhibit Hall
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## [65.09] Three-Dimensional AMR-MHD Simulations of Protostellar Core Collapse and Fragmentation

*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

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