Can magnetized turbulence set the mass scale of stars?

Abstract

Understanding the evolution of self-gravitating, isothermal, magnetized gas is crucial for star formation, as these physical processes have been postulated to set the initial mass function (IMF). We present a suite of isothermal magnetohydrodynamic (MHD) simulations using the GIZMO code that follow the formation of individual stars in giant molecular clouds (GMCs), spanning a range of Mach numbers found in observed GMCs (⁠M∼10−50⁠). As in past works, the mean and median stellar masses are sensitive to numerical resolution, because they are sensitive to low-mass stars that contribute a vanishing fraction of the overall stellar mass. The mass-weighted median stellar mass M₅₀ becomes insensitive to resolution once turbulent fragmentation is well resolved. Without imposing Larson-like scaling laws, our simulations find M₅₀∝∼M₀M⁻³α_(turb)SFE^(1/3) for GMC mass M₀, sonic Mach number M⁠, virial parameter α_(turb), and star formation efficiency SFE = M⋆/M₀. This fit agrees well with previous IMF results from the RAMSES, ORION2, and SPHNG codes. Although M₅₀ has no significant dependence on the magnetic field strength at the cloud scale, MHD is necessary to prevent a fragmentation cascade that results in non-convergent stellar masses. For initial conditions and SFE similar to star-forming GMCs in our Galaxy, we predict M₅₀ to be >20M⊙⁠, an order of magnitude larger than observed (⁠∼2M⊙⁠), together with an excess of brown dwarfs. Moreover, M₅₀ is sensitive to initial cloud properties and evolves strongly in time within a given cloud, predicting much larger IMF variations than are observationally allowed. We conclude that physics beyond MHD turbulence and gravity are necessary ingredients for the IMF.