  How do stars that are more massive than the Sun form, and thus how is the
stellar initial mass function (IMF) established? Such intermediate- and
high-mass stars may be born from relatively massive pre-stellar gas cores,
which are more massive than the thermal Jeans mass. The Turbulent Core
Accretion model invokes such cores as being in approximate virial equilibrium
and in approximate pressure equilibrium with their surrounding clump medium.
Their internal pressure is provided by a combination of turbulence and magnetic
fields. Alternatively, the Competitive Accretion model requires strongly
sub-virial initial conditions that then lead to extensive fragmentation to the
thermal Jeans scale, with intermediate- and high-mass stars later forming by
competitive Bondi-Hoyle accretion. To test these models, we have identified
four prime examples of massive (~100Msun) clumps from mid-infrared extinction
mapping of infrared dark clouds (IRDCs). Fontani et al. found high deuteration
fractions of N2H+ in these objects, which are consistent with them being
starless. Here we present ALMA observations of these four clumps that probe the
N2D+(3-2) line at 2.3" resolution. We find six N2D+ cores and determine their
dynamical state. Their observed velocity dispersions and sizes are broadly
consistent with the predictions of the Turbulent Core model of
self-gravitating, magnetized (with Alfven Mach number m_A~1) and virialized
cores that are bounded by the high pressures of their surrounding clumps.
However, in the most massive cores, with masses up to ~60Msun, our results
suggest that moderately enhanced magnetic fields (so that m_A~0.3) may be
needed for the structures to be in virial and pressure equilibrium.
Magnetically regulated core formation may thus be important in controlling the
formation of massive cores, inhibiting their fragmentation, and thus helping to
establish the stellar IMF.
