The formation of star clusters - II. 3D simulations of magnetohydrodynamic turbulence in molecular clouds
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abstract
(Abridged) We present a series of decaying turbulence simulations that
represent a cluster-forming clump within a molecular cloud, investigating the
role of magnetic fields on the formation of potential star-forming cores. We
present an exhaustive analysis of numerical data from these simulations that
includes a compilation of all of the distributions of physical properties that
characterize bound cores - including their masses, radii, mean densities,
angular momenta, spins, magnetizations, and mass-to-flux ratios. We also
present line maps of our models that can be compared with observations. Our
simulations range between 5-30 Jeans masses of gas, and are representative of
molecular cloud clumps with masses between 100-1000 solar masses. The cores
have mass-to-flux ratios that are generally less than that of the original
cloud, and so a cloud that is initially highly supercritical can produce cores
that are slightly supercritical, similar to that seen by Zeeman measurements of
molecular cloud cores. Clouds that are initially only slightly supercritical
will instead collapse along the field lines into sheets, and the cores that
form as these sheets fragment have a different mass spectrum than what is
observed. The spin rates of these cores suggests that subsequent fragmentation
into multiple systems is likely. The sizes of the bound cores that are produced
are typically 0.02-0.2 pc and have densities in the range 10^4-10^5 cm^{-3} in
agreement with observational surveys. Finally, our numerical data allow us to
test theoretical models of the mass spectrum of cores, such as the turbulent
fragmentation picture of Padoan-Nordlund. We find that while this model gets
the shape of the core mass spectrum reasonably well, it fails to predict the
peak mass in the core mass spectrum.
