The infrared dark cloud (IRDC) G028.23-00.19 hosts a massive (1500 M ⊙), cold (12 K), and 3.6–70 μm IR dark clump (MM1) that has the potential to form high-mass stars. We observed this prestellar clump candidate with the Submillimeter Array (∼3.″5 resolution) and Jansky Very Large Array (∼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 five cores with masses ≤15 M ⊙. 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 (ΔV nt/ΔV th ∼ 1.1–1.8). The cores are not highly supersonic as some theories of high-mass star formation predict. The embedded cores are four to seven times more massive than the clump thermal Jeans mass and the most massive core (SMA1) is nine 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.
