Observational identification of molecular cloud collapse has been one of the
most challenging tasks in the field of star formation. The main
difficulty of detecting infall arises from the small scale of the collapsing zone, placement of molecular gas with respect to the central source,
and confusion
by the competing processes such as molecular outflows and rotation.
In my thesis studies, we established an observational paradigm
of gravitational
collapse in massive molecular cloud cores (<A HREF="./paper/w51nh3.ps">
Zhang & Ho 1997</A>; <A HREF="./paper/w51cs.ps"> Zhang, Ho
& Ohashi 1997 </A>).
Through a series of experiments with
radio interferometers,
we identified perhaps the best
candidate of infall in massive cores. The spectroscopic
signatures of inward motions are consistently present in molecular
transitions of
NH3, C18O, CS, CH3CN, HC3N, CH3OCH3 and HCOOCH3.
The inverse P-Cygni
line profiles, i.e., the blue-shifted emission and red-shifted absorption
indicate inward motions toward the central star. Furthermore, the
velocity pattern across the core agrees well with the expected signature
of radial motions in the core. The infall speed remains constant throughout
the infall zone which has a density profile of r^{-2},
contrary to theoretical predictions (r^{-1.5}) of collapse in low mass
cores. The inward motion is accompanied
by accretion flows which spin up (omega ~ r^{-1})
(e.g. Shu 1977) toward the center of the cloud core.
These observations provide useful constraints
to theoretical models of massive core collapse.