  High-mass stars have generally been assumed to accrete most of their mass
while already contracted onto the main sequence, but this hypothesis has not
been observationally tested. We present ALMA observations of a 3 x 1.5 pc area
in the W51 high-mass star-forming complex. We identify dust continuum sources
and measure the gas and dust temperature through both rotational diagram
modeling of CH3OH and brightness-temperature-based limits. The observed region
contains three high-mass YSOs that appear to be at the earliest stages of their
formation, with no signs of ionizing radiation from their central sources. The
data reveal high gas and dust temperatures (T > 100 K) extending out to about
5000 AU from each of these sources. There are no clear signs of disks or
rotating structures down to our 1000 AU resolution. The extended warm gas
provides evidence that, during the process of forming, these high-mass stars
heat a large volume and correspondingly large mass of gas in their
surroundings, inhibiting fragmentation and therefore keeping a large reservoir
available to feed from. By contrast, the more mature massive stars that
illuminate compact H ii regions have little effect on their surrounding dense
gas, suggesting that these main sequence stars have completed most or all of
their accretion. The high luminosity of the massive protostars (L > 10^4 solar
luminosities), combined with a lack of centimeter continuum emission from these
sources, implies that they are not on the main sequence while they accrete the
majority of their mass; instead, they may be bloated and cool.
