Nobeyama 45 m Cygnus-X CO Survey. II. Physical Properties of C18O Clumps

We report the statistical physical properties of the C¹⁸O(J = 1-0) clumps present in a prominent cluster-forming region, Cygnus X, using the data set obtained by the Nobeyama 45 m radio telescope. This survey covers 9 deg² of the northern and southern regions of Cygnus X, and, in total, 174 C¹⁸O clumps are identified using the dendrogram method. Assuming a distance of 1.4 kpc, these clumps have radii of 0.2-1 pc, velocity dispersions of <2.2 km s^-1, gas masses of 30-3000 M _⊙, and H₂ densities of (0.2-5.5) × 10⁴ cm^-3. We confirm that the C¹⁸O clumps in the northern region have a higher H₂ density than those in the southern region, supporting the existence of a difference in the evolutionary stages, consistent with the star-formation activity of these regions. The difference in the clump properties of the star-forming and starless clumps is also confirmed by the radius, velocity dispersion, gas mass, and H₂ density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C¹⁸O clump mass function shows two spectral index components, α = -1.4 in 55-140 M _⊙ and α = -2.1 in >140 M _⊙, which are consistent with the low- and intermediate-mass parts of the Kroupa’s initial mass function. The spectral index of the star-forming clumps >140 M _⊙ is consistent with that of the starless clumps ranging from 55-140 M _⊙, suggesting that the latter will evolve into star-forming clumps while retaining the gas accretion. Assuming a typical star-formation efficiency of molecular clumps (10%), about 10 C¹⁸O clumps having a gas mass of >10³ M _⊙ will evolve into open clusters containing one or more OB stars.