The Galactic Census of High- and Medium-mass Protostars. III. 12CO Maps and Physical Properties of Dense Clump Envelopes and Their Embedding GMCs

We report the second complete molecular line data release from the Census of High- and Medium-mass Protostars (CHaMP), a large-scale, unbiased, uniform mapping survey at sub-parsec resolution, of millimeter-wave line emission from 303 massive, dense molecular clumps in the Milky Way. This release is for all ¹²CO J = 1 \to 0 emission associated with the dense gas, the first from Phase II of the survey, which includes ¹²CO, ¹³CO, and C¹⁸O. The observed clump emission traced by both ¹²CO and HCO⁺ (from Phase I) shows very similar morphology, indicating that, for dense molecular clouds and complexes of all sizes, parsec-scale clumps contain Ξ ∼ 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 of 10⁹ m^-3 is {ξ }₉ ≳ 50%. This suggests that parsec-scale clumps may be the basic building blocks of the molecular interstellar medium, rather than the standard GMC concept. Using ¹²CO 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}_{¹²{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 that only terminally engage in vigorous massive star formation, supporting other evidence along these lines that was previously published.