Lowering the Characteristic Mass of Cluster Stars by Magnetic Fields and
  Outflow Feedback
  Magnetic fields are generally expected to increase the characteristic mass of
stars formed in stellar clusters, because they tend to increase the effective
Jeans mass. We test this expectation using adaptive mesh refinement (AMR)
magnetohydrodynamic simulations of cluster formation in turbulent magnetized
clumps of molecular clouds, treating stars as accreting sink particles. We find
that, contrary to the common expectation, a magnetic field of strength in the
observed range decreases, rather than increases, the characteristic stellar
mass. It (1) reduces the number of intermediate-mass stars that are formed
through direct turbulent compression, because sub-regions of the clump with
masses comparable to those of stars are typically magnetically subcritical and
cannot be compressed directly into collapse, and (2) increases the number of
low-mass stars that are produced from the fragmentation of dense filaments. The
filaments result from mass accumulation along the field lines. In order to
become magnetically supercritical and fragment, the filament must accumulate a
large enough column density (proportional to the field strength), which yields
a high volume density (and thus a small thermal Jeans mass) that is conducive
to forming low-mass stars. We find, in addition, that the characteristic
stellar mass is reduced further by outflow feedback. The conclusion is that
both magnetic fields and outflow feedback are important in shaping the stellar
initial mass function (IMF).
