Detecting Infall in High-Mass Protostellar Objects
  The role of accretion disks in the formation of low-mass stars has been well
assessed by means of high angular resolution observations at various
wavelengths. These findings confirm the prediction that conservation of angular
momentum during the collapse leading to the formation of a star is bound to
produce flattening and rotation of the collapsing core. What about high-mass
stars? At present, several authors have reported on detections of disks around
high-mass YSOs. Notwithstanding these important results, the presence of disks
rotating about high-mass stars is not sufficient by itself to prove
unambiguously the accretion model: what is needed is iron-clad evidence of
infall. Such evidence is very difficult to find, as the free-fall velocity
becomes significant only very close to the accreting star, i.e., over a region
of a few 0.01 pc ($\sim$2000 au), which is very difficult to access and
disentangle from the surrounding quiescent or rotating material. In this
chapter we discuss how to characterize the infall of material in a sample of 36
high-mass accretion disk candidates covering a broad range of luminosities,
from 10$^3$ $L_\odot$ to 10$^6$ $L_\odot$, compiled by Beltr\'an & de Wit
(2016) with the next generation Very Large Array (ngVLA).
