The Hot and Dynamic Birth of Massive Stars from the ngVLA Perspective
  The Next Generation Very Large Array (ngVLA) has excellent capabilities to
unveil various dynamical and chemical processes in massive star formation at
the unexplored innermost regions. Based on the recent observations of ALMA/VLA
as well as theoretical predictions, we propose several intriguing topics in
massive star formation from the perspective of the ngVLA. In the disk scale of
$\lesssim$ 100 au around massive protostars, dust grains are expected to be
destructed/sublimated because the physical conditions of temperature, shocks,
and radiation are much more intense than those in the envelopes, which are
typically observed as hot cores. The high sensitivity and resolution of the
ngVLA will enable us to detect the gaseous refractories released by dust
destruction, e.g., SiO, NaCl, and AlO, which trace disk kinematics and give new
insights into the metallic elements in star-forming regions, i.e.,
astromineralogy. The multi-epoch survey by the ngVLA will provide demographics
of forming massive multiples with separations of $\lesssim$ 10 au with their
proper motion. Combining with observations of refractory molecular lines and
hydrogen recombination lines, we can reproduce the three-dimensional orbital
motions of massive proto-binaries. Moreover, the 1-mas resolution of the ngVLA
could possibly take the first-ever picture of the photospheric surface of an
accreting protostar, if it is bloated to the au scale by the high accretion
rates of mass and thermal energy.
