Gaseous CO Abundance---An Evolutionary Tracer for Molecular Clouds
  Planck cold clumps are among the most promising objects to investigate the
initial conditions of the evolution of molecular clouds. In this work, by
combing the dust emission data from the survey of Planck satellite with the
molecular data of $^{12}$CO/$^{13}$CO (1-0) lines from observations with the
Purple Mountain Observatory (PMO) 13.7 m telescope, we investigate the CO
abundance, CO depletion and CO-to-H$_{2}$ conversion factor of 674 clumps in
the early cold cores (ECC) sample. The median and mean values of the CO
abundance are 0.89$\times10^{-4}$ and 1.28$\times10^{-4}$, respectively. The
mean and median of CO depletion factor are 1.7 and 0.9, respectively. The
median value of $X_{CO-to-H_{2}}$ for the whole sample is $2.8\times10^{20}$
cm$^{-2}$K$^{-1}$km$^{-1}$ s. The CO abundance, CO depletion factor and
CO-to-H$_{2}$ conversion factor are strongly (anti-)correlated to other
physical parameters (e.g. dust temperature, dust emissivity spectral index,
column density, volume density and luminosity-to-mass ratio). To conclude, the
gaseous CO abundance can be used as an evolutionary tracer for molecular
clouds.
