Nobeyama 45-m Cygnus X CO Survey: II Physical Properties of ${\mathrm{C}}^{18}\mathrm{O}$ Clumps

Abstract

We report the statistical physical properties of the C${}^{18}$O($J=1-0$) clumps present in a prominent cluster-forming region, Cygnus X, using the dataset obtained by the Nobeyama 45-m radio telescope. This survey covers 9 deg${}^2$ of the north and south regions of Cygnus X, and totally 174 C${}^{18}$O 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 $<2.2\mathrm{km}{\mathrm{s}}^{-1}$, gas masses of 30-3000 $M_{\odot }$, and H${}_2$ densities of (0.2-5.5)$\times 10^4\mathrm{c}{\mathrm{m}}^{-3}$. We confirm that the C${}^{18}$O clumps in the north region have a higher H${}_2$ density than those in the south region, supporting the existence of a difference in the evolution 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 H${}_2$ density. The average virial ratio of 0.3 supports that these clumps are gravitationally bound. The C${}^{18}$O clump mass function shows two spectral index components, $\alpha =-1.4$ in 55-140 $M_{\odot }$ and $\alpha =-2.1$ in $>140M_{\odot }$, which are consistent with the low- and intermediate-mass parts of the Kroupa's initial mass function. The spectral index in the star-forming clumps in $>140M_{\odot }$ is consistent with that of the starless clumps in 55-140 $M_{\odot }$, 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 ten C${}^{18}$O clumps having a gas mass of $>10^3{M}_{\odot }$ will evolve into open clusters containing one or more OB stars.

20 pages, 7 figures, accepted for publication in the Astrophysical Journal