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. Oursimulations include an extensive set of physical processes - magnetohydrodynamics, radiativetransfer, and protostellar outflows - and span a wide range of virial parameters and magneticfield strengths. Comparing the outcomes of our simulations to observations, we find thatsimulations remaining close to virial balance throughout their history produce star formationefficiencies and initial mass function (IMF) peaks that are stable in time and in reasonableagreement with observations. Our results indicate that small-scale dissipation effects nearthe 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 largescaleforcing or by strong magnetic fields that inhibit collapse. In contrast, simulations thatleave virial balance and undergo runaway collapse form stars too efficiently and produce anIMF that becomes increasingly top heavy with time. In all cases, we find that the competitionbetween magnetic flux advection towards the protostar and outward advection due to magneticinterchange instabilities, and the competition between turbulent amplification and reconnectionclose to newly formed protostars renders the local magnetic field structure insensitive to thestrength of the large-scale field, ensuring that radiation is always more important than magneticsupport in setting the fragmentation scale and thus the IMFpeak mass. The statistics ofmultiplestellar systems are similarly insensitive to variations in the initial conditions and generally agreewith observations within the range of statistical uncertainty.