  Methods: We observed the high-mass hot core region G351.77-0.54 with ALMA and
more than 16km baselines.
  Results: At a spatial resolution of 18/40au (depending on the distance), we
identify twelve sub-structures within the inner few thousand au of the region.
The brightness temperatures are high, reaching values greater 1000K,
signposting high optical depth toward the peak positions. Core separations vary
between sub-100au to several 100 and 1000au. The core separations and
approximate masses are largely consistent with thermal Jeans fragmentation of a
dense gas core. Due to the high continuum optical depth, most spectral lines
are seen in absorption. However, a few exceptional emission lines are found
that most likely stem from transitions with excitation conditions above1000K.
Toward the main continuum source, these emission lines exhibit a velocity
gradient across scales of 100-200au aligned with the molecular outflow and
perpendicular to the previously inferred disk orientation. While we cannot
exclude that these observational features stem from an inner hot accretion
disk, the alignment with the outflow rather suggests that it stems from the
inner jet and outflow region. The highest-velocity features are found toward
the peak position, and no Hubble-like velocity structure can be identified.
Therefore, these data are consistent with steady-state turbulent entrainment of
the hot molecular gas via Kelvin-Helmholtz instabilities at the interface
between the jet and the outflow.
  Conclusions: Resolving this high-mass star-forming region at sub-50au scales
indicates that the hierarchical fragmentation process in the framework of
thermal Jeans fragmentation can continue down to the smallest accessible
spatial scales. Velocity gradients on these small scales have to be treated
cautiously and do not necessarily stem from disks, but may be better explained
with outflow emission.
