The TOP-SCOPE Survey of Planck Galactic Cold Clumps: Molecular Gas Properties

We surveyed 2008 Planck Galactic Cold Clumps (PGCCs) in 12 CO and 13 CO J = 1–0 lines using the Taeduk Radio Astronomy Observatory 14 m telescope’s multibeam receiver. We detected 2784 ( 12 CO) and 2291 ( 13 CO) velocity components, their closely correlated centroid velocities suggest that 12 CO and 13 CO generally trace kinematically associated gas. PGCCs have low excitation temperatures (mean ∼10 K), mean 13 CO optical depth ∼0.5, and mean 13 CO-derived H 2 column density 4.3 × 10 21 cm −2 . Gas–dust correlations are moderate, with N 13 CO more tightly correlated with the dust-derived H 2 column density from the PGCC catalog than I 12 CO . Colder PGCCs tend to have a higher CO-to-H 2 conversion factor ( X CO ) and [H 2 ]/[ 13 CO] ratio. X CO increases clearly with the dust-derived H 2 column density, consistent with enhanced CO freeze-out in high-column-density gas. Supersonic nonthermal motions are widespread: the Mach number derived from 13 CO has a mean of 4.3 and a median of 3.6, increasing slightly with dust-derived H 2 column density. Overall, PGCCs are cold but dynamically active, serving as a valuable laboratory for studying the initial conditions of star formation.