The TOP-SCOPE Survey of Planck Galactic Cold Clumps: Molecular gas properties

Abstract

We surveyed 2008 Planck Galactic Cold Clumps (PGCCs) in ${}^{12}\mathrm{CO}$ and ${}^{13}\mathrm{CO}$ $J=1$--0 lines using the Taeduk Radio Astronomy Observatory (TRAO) 14 m telescope's multi-beam receiver. We detected 2784 (${}^{12}\mathrm{CO}$) and 2291 (${}^{13}\mathrm{CO}$) 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 $\sim$10 K), mean ${}^{13}\mathrm{CO}$ optical depth $\sim$0.5, and mean ${}^{13}\mathrm{CO}$-derived H${}_2$ column density $4.3\times 10^{21}$~cm${}^{-2}$. Gas--dust correlations are moderate, with $N_{{}^{13}\mathrm{CO}}$ more tightly correlated with the dust-derived H${}_2$ column density from the PGCC catalog than $I_{{}^{12}\mathrm{CO}}$. Colder PGCCs tend to have higher CO-to-H${}_2$ conversion factor ($X_{\mathrm{CO}}$) and $[{\mathrm{H}}_2]/{[}^{13}\mathrm{CO}]$ ratio. $X_{\mathrm{CO}}$ 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 ${}^{13}\mathrm{CO}$ 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.

21 pages, 14 figures, Accepted by AJ