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 (TRAO) 14 m telescope's multi-beam 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 about 10 K), mean 13 CO optical depth about 0.5, and mean 13 CO-derived H 2 column density 4.3 times10 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 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 non-thermal 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.