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

Abstract

Abstract 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.