2018ApJ...866...19B

We present ¹²CO, ¹³CO, and C¹⁸O data as the next major release for the CHaMP project, an unbiased sample of Galactic molecular clouds in l = 280°-300°. From a radiative transfer analysis, we self-consistently compute 3D cubes of optical depth, excitation temperature, and column density for ∼300 massive clumps, and update the I_^12CO-dependent CO - H₂ conversion law of Barnes et al. For N ∝ I^p, we find p = 1.92 ± 0.05 for the velocity-resolved conversion law aggregated over all clumps. A practical, integrated conversion law is N_^12CO= (4.0 ± 0.3) x 10¹⁹ m^–2 I_^{12CO1.27±0.02}, confirming an overall 2x higher total molecular mass for Milky Way clouds, compared to the standard X factor. We use these laws to compare the kinematics of clump interiors with their foreground ¹²CO envelopes, and find evidence that most clumps are not dynamically uniform: irregular portions seem to be either slowly accreting onto the interiors, or dispersing from them. We compute the spatially resolved mass accretion/dispersal rate across all clumps, and map the local flow timescale. While these flows are not clearly correlated with clump structures, the inferred accretion rate is a statistically strong function of the local mass surface density Σ, suggesting near-exponential growth or loss of mass over effective timescales ∼30-50 Myr. At high enough Σ, accretion dominates, suggesting gravity plays an important role in both processes. If confirmed by numerical simulations, this sedimentation picture would support arguments for long clump lifetimes mediated by pressure confinement, with a terminal crescendo of star formation, suggesting a resolution to the 40 year old puzzle of the dynamical state of molecular clouds and their low star formation efficiency.