A CO SURVEY OF REGIONS AROUND 3J OPEN CLUSTERS .2. PHYSICAL-PROPERTIES OF CATALOGED MOLECULAR CLOUDS

Physical properties of the 148 molecular clouds found in a systematic survey of CO emission from regions around 34 young open clusters in the Milky Way are studied. Sources of bias and completeness of samples of clouds drawn from the catalog are discussed. The cloud size spectrum is found to be of the form N(R > R0) ∝ Ro-1.6 (8 pc < R0 < 20 pc), and the mass spectrum is given by N(M > M0) ∝ M0-0.65 (2.0 × 103 M⊙ < M0 < 1.3 × 105 M⊙). From nonlinear least-squares fits which take into account errors in observed parameters, two power-law scaling relations are measured: M ∝ R3 and ΔV ∝ R0.6, where ΔV is the CO line width. The mass-radius relation implies that clouds of all sizes larger than a few pc have approximately the same mean volume density, about 20 H2 cm-3 assuming a standard CO intensity to H2 column density conversion factor. The clouds are not in virial equilibrium both in the sense that the observed CO lines are wider than Vvir = (3GM/5R)1/2, and in that ΔV/Vvir is a decreasing function of R. Molecular clouds associated with open clusters younger than 10 Myr are larger and more massive than, but have the same mean density as, the clouds associated with older clusters. All the CO observations can be understood in terms of a model in which molecular clouds are thought of as ensembles of dense clumps. On the basis of such a model, it is demonstrated that molecular clouds are perturbed on a time scale short compared to the time required for them to reestablish virial equilibrium. The least certain parameter of the model characterizes the degree to which colliding clumps "remember" their trajectories; a prediction is made of its value based on the determination that clouds are not in equilibrium. The relatively small local dark clouds found in the survey of open cluster regions may resemble the clumps in larger clouds.