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~ { } s ^ { -1 } } , gas masses of 30–3000 M _ { \sun } , and H _ { 2 } densities of ( 0.2–5.5 ) \times 10 ^ { 4 } ~ { } \mathrm { cm ^ { -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 _ { \sun } and \alpha = -2.1 in > 140 ~ { } M _ { \sun } , 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 > 140 ~ { } M _ { \sun } is consistent with that of the starless clumps in 55–140 M _ { \sun } , 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 _ { \sun } will evolve into open clusters containing one or more OB stars .