  The Infrared Dark Cloud (IRDC) G028.23-00.19 hosts a massive (1,500 Msun),
cold (12 K), and 3.6-70 um IR dark clump (MM1) that has the potential to form
high-mass stars. We observed this prestellar clump candidate with the SMA
(~3.5" resolution) and JVLA (~2.1" resolution) in order to characterize the
early stages of high-mass star formation and to constrain theoretical models.
Dust emission at 1.3 mm wavelength reveals 5 cores with masses <15 Msun. None
of the cores currently have the mass reservoir to form a high-mass star in the
prestellar phase. If the MM1 clump will ultimately form high-mass stars, its
embedded cores must gather a significant amount of additional mass over time.
No molecular outflows are detected in the CO (2-1) and SiO (5-4) transitions,
suggesting that the SMA cores are starless. By using the NH3 (1,1) line, the
velocity dispersion of the gas is determined to be transonic or mildly
supersonic (DeltaV_nt}/DeltaV_th ~1.1-1.8). The cores are not highly supersonic
as some theories of high-mass star formation predict. The embedded cores are 4
to 7 times more massive than the clump thermal Jeans mass and the most massive
core (SMA1) is 9 times less massive than the clump turbulent Jeans mass. These
values indicate that neither thermal pressure nor turbulent pressure dominates
the fragmentation of MM1. The low virial parameters of the cores (0.1-0.5)
suggest that they are not in virial equilibrium, unless strong magnetic fields
of ~1-2 mG are present. We discuss high-mass star formation scenarios in a
context based on IRDC G028.23-00.19, a study case believed to represent the
initial fragmentation of molecular clouds that will form high-mass stars.
