  We present new JVLA observations of the high-mass cluster-forming region W51A
from 2 to 16 GHz with resolution ${\theta}_{fwhm} \approx$ 0.3 - 0.5". The data
reveal a wealth of observational results: (1) Currently-forming, very massive
(proto-O) stars are traced by o-H2CO $2_{1,1}-2_{1,2}$ emission, suggesting
that this line can be used efficiently as a massive protostar tracer. (2) There
is a spatially distributed population of $\sim$mJy continuum sources, including
hypercompact H ii regions and candidate colliding wind binaries, in and around
the W51 proto-clusters. (3) There are two clearly detected protoclusters, W51e
and W51 IRS2, that are gas-rich but may have most of their mass in stars within
their inner $\sim$ 0.05 pc. The majority of the bolometric luminosity in W51
most likely comes from a third population of OB stars between these clusters.
The presence of a substantial population of exposed O-stars coincident with a
population of still-forming massive stars, along with a direct measurement of
the low mass loss rate via ionized gas outflow from W51 IRS2, together imply
that feedback is ineffective at halting star formation in massive
protoclusters. Instead, feedback may shut off the large-scale accretion of
diffuse gas onto the W51 protoclusters, implying that they are evolving towards
a state of gas exhaustion rather than gas expulsion. Recent theoretical models
predict gas exhaustion to be a necessary step in the formation of
gravitationally bound stellar clusters, and our results provide an
observational validation of this process.
