The Effects of Magnetic Fields and Protostellar Feedback on Low-mass
  Cluster Formation
  We present a large suite of simulations of the formation of low-mass star
clusters. Our simulations include an extensive set of physical processes --
magnetohydrodynamics, radiative transfer, and protostellar outflows -- and span
a wide range of virial parameters and magnetic field strengths. Comparing the
outcomes of our simulations to observations, we find that simulations remaining
close to virial balance throughout their history produce star formation
efficiencies and initial mass function (IMF) peaks that are stable in time and
in reasonable agreement with observations. Our results indicate that
small-scale dissipation effects near the protostellar surface provide a
feedback loop for stabilizing the star formation efficiency. This is true
regardless of whether the balance is maintained by input of energy from large
scale forcing or by strong magnetic fields that inhibit collapse. In contrast,
simulations that leave virial balance and undergo runaway collapse form stars
too efficiently and produce an IMF that becomes increasingly top-heavy with
time. In all cases we find that the competition between magnetic flux advection
toward the protostar and outward advection due to magnetic interchange
instabilities, and the competition between turbulent amplification and
reconnection close to newly-formed protostars renders the local magnetic field
structure insensitive to the strength of the large-scale field, ensuring that
radiation is always more important than magnetic support in setting the
fragmentation scale and thus the IMF peak mass. The statistics of multiple
stellar systems are similarly insensitive to variations in the initial
conditions and generally agree with observations within the range of
statistical uncertainty.
