  Context: The importance of magnetic fields at the onset of star formation
related to the early fragmentation and collapse processes is largely unexplored
today. Aims: We want to understand the magnetic field properties at the
earliest evolutionary stages of high-mass star formation. Methods: The Atacama
Large Millimeter Array is used at 1.3mm wavelength in full polarization mode to
study the polarized emission and by that the magnetic field morphologies and
strengths of the high-mass starless region IRDC18310-4. Results: The polarized
emission is clearly detected in four sub-cores of the region. In general it
shows a smooth distribution, also along elongated cores. Estimating the
magnetic field strength via the Davis-Chandrasekhar-Fermi method and following
a structure function analysis, we find comparably large magnetic field
strengths between ~0.6 and 3.7mG. Comparing the data to spectral line
observations, the turbulent-to-magnetic energy ratio is low, indicating that
turbulence does not significantly contribute to the stability of the gas clump.
A mass-to-flux ratio around the critical value 1.0 - depending on column
density - indicates that the region starts to collapse which is consistent with
the previous spectral line analysis of the region. Conclusions: While this
high-mass region is collapsing and thus at the verge of star formation, the
high magnetic field values and the smooth spatial structure indicate that the
magnetic field is important for the fragmentation and collapse process. This
single case study can only be the starting point for larger sample studies of
magnetic fields at the onset of star formation.
