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We present measurements of the star formation efficiency (SFE) in 3D
numerical simulations of driven turbulence in supercritical,
ideal-MHD, and non-magnetic regimes, characterized by their mean
normalized mass-to-flux ratio $\mu$, all with 64 Jeans masses and
similar rms Mach numbers ($\sim 10$). In most cases, the moderately
supercritical runs with $\mu = 2.8$ have significantly lower SFEs than
the non-magnetic cases, being comparable to observational estimates
for whole molecular clouds ($\le$ 5\% over 4 Myr). Also, as the mean
field is increased, the number of collapsed objects decreases, and the
median mass of the collapsed objects increases. However, the largest
collapsed-object masses systematically occur in the weak-field case
$\mu = 8.8$. The high-density tails of the density histograms in the
simulations are depressed as the mean magnetic field strength is
increased. This suggests that the smaller numbers and larger masses
of the collapsed objects in the magnetic cases may be due to a greater
scarcity and lower mean densities (implying larger Jeans masses) of
the collapse candidates. In this scenario, the effect of a weak field
is to reduce the probability of a core reaching its thermal Jeans
mass, even if it is supercritical. We thus suggest that the SFE may
be monotonically reduced as the field strength increases from zero to
subcritical values, rather than there being a discontinuous transition
between the sub- and supercritical regimes, and that a crucial
question to address is whether the turbulence in molecular clouds is
driven or decaying, with current observational and theoretical
evidence favoring (albeit inconclusively) the driven regime.
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