JVLA images of W51A

The data reveal a wealth of observational results: (1) Currently-forming, very massive (proto-O) stars are traced by o-H_2_CO 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 <~mJy continuum sources, including hypercompact HII 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 <~0.05pc. 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.