The Galactic Census of High- and Medium-mass Protostars. III $^{12}$CO Maps and Physical Properties of Dense Clump Envelopes and their Embedding GMCs

TL;DR

This paper presents a comprehensive molecular line survey of 303 dense molecular clumps in the Milky Way, revealing that parsec-scale clumps are fundamental units of the molecular ISM and providing insights into their physical properties and stability.

Contribution

It offers the second data release from the CHaMP survey, including $^{12}$CO, $^{13}$CO, and C$^{18}$O maps, and analyzes the physical state and stability of dense molecular clumps.

Findings

Most mass (~75%) is in parsec-scale clumps, not extended structures.

Clumps are closer to virial equilibrium, indicating potential longevity.

Alternative X-factor conversions yield more physically consistent properties.

Abstract

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.