2013ApJ...775L...2L

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 the Planck satellite with the molecular data of ¹²CO/¹³CO/C¹⁸O (1-0) lines from observations with the Purple Mountain Observatory 13.7 m telescope, we investigate the CO abundance, CO depletion, and CO-to-H₂ conversion factor of 674 clumps in the early cold cores sample. The median and mean values of the CO abundance are 0.89x10^–4 and 1.28x10^–4, respectively. The mean and median of CO depletion factor are 1.7 and 0.9, respectively. The median value of X_Co–toH2_ for the whole sample is 2.8x10²⁰/cm2.K^–1.km^–1 s. The CO abundance, CO depletion factor, and CO-to-H₂ 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.