The Galactic Census of High- and Medium-mass Protostars. III $^{12}$CO
  Maps and Physical Properties of Dense Clump Envelopes and their Embedding
  GMCs
  We report the second complete molecular line data release from the {\em
Census of High- and Medium-mass Protostars} (CHaMP), a large-scale, unbiased,
uniform mapping survey at sub-parsec resolution, of mm-wave line emission from
303 massive, dense molecular clumps in the Milky Way. This release is for all
$^{12}$CO $J$=1$\rightarrow$0 emission associated with the dense gas, the first
from Phase II of the survey, which includes $^{12}$CO, $^{13}$CO, and
C$^{18}$O. The observed clump emission traced by both $^{12}$CO and HCO$^+$
(from Phase I) shows very similar morphology, indicating that, for dense
molecular clouds and complexes of all sizes, parsec-scale clumps contain $\Xi$
~ 75% of the mass, while only 25% of the mass lies in extended (>~ 10 pc) or
"low density" components in these same areas. The mass fraction of all gas
above a density 10$^9$ m$^{-3}$ is $\xi_9$ >~ 50%. This suggests that
parsec-scale clumps may be the basic building blocks of the molecular ISM,
rather than the standard GMC concept. Using $^{12}$CO emission, we derive
physical properties of these clumps in their entirety, and compare them to
properties from HCO$^+$, tracing their denser interiors. We compare the
standard X-factor converting $I_{CO}$ to $N_{H_2}$ with alternative
conversions, and show that only the latter give whole-clump properties that are
physically consistent with those of their interiors. We infer that the clump
population is systematically closer to virial equilibrium than when considering
only their interiors, with perhaps half being long-lived (10s of Myr),
pressure-confined entities which only terminally engage in vigorous massive
star formation, supporting other evidence along these lines previously
published.
