Lack of high-mass prestellar cores in the starless MDCs of NGC6334
  Two families of models compete to explain the formation of high-mass stars.
The quasi-static models predict the existence of high-mass pre-stellar cores
sustained by a high degree of turbulence while competitive accretion models
predict that high-mass proto-stellar cores evolve from low/intermediate mass
proto-stellar cores in dynamic environments. We present ALMA (1.4 mm continuum
emission and $^{12}$CO emission line) and MOPRA (HCO$^{+}$, H$^{13}$CO$^{+}$
and N$_2$H$^+$ molecular line emissions) observations of a sample of 9 starless
massive dense cores (MDCs) discovered in a recent Herschel/HOBYS study that
have masses and sizes ($\sim$110 M$_\odot$ and $r\sim$0.1 pc, respectively)
similar to the initial conditions used in the quasi-static models. The MOPRA
molecular line features show that 3 of the starless MDCs are subvirialized with
$\alpha_{\rm vir}\sim$0.35, and 4 MDCs show sign of collapse. Our ALMA
observations, on the other hand, show very little fragmentation within the MDCs
whereas the observations resolve the Jeans length ($\lambda_{\rm
Jeans}\sim$0.03 pc) and are sensitive to the Jeans mass (M$_{\rm
Jeans}\sim$0.65 M$_\odot$) in the 9 starless MDCs. Only two of the starless
MDCs host compact continuum sources, whose fluxes correspond to $<3$ M$_\odot$
fragments. Therefore the mass reservoir of the MDCs has not yet been accreted
onto compact objects, and most of the emission is filtered out by the
interferometer. These observations do not support the quasi-static models for
high-mass star formation since no high-mass pre-stellar core is found in
NGC6334. The competitive accretion models, on the other hand, predict a level
of fragmentation much higher than what we observe.
