| Abstract
| - We present an analysis of the large-scale molecular cloud structure and of the stability of clumpy structures in nearby molecular clouds. In our recent work, we identified a structural transition in molecular clouds by studying the probability distributions of their gas column densities. In this paper, we further examine the nature of this transition. The transition takes place at the visual extinction of \hbox{$A_\mathrm{V}^\mathrm{tail} = 2{-}4$}AVtail=2−4 mag, or equivalently, at Σ tail ≈ 40−80 M⊙ pc -2. The clumps identified above this limit have wide ranges of masses and sizes, but a remarkably constant mean volume density of \hbox{$overline{n} \approx 10^3$}n≈103 cm -3. This is 5−10 times higher than the density of the medium surrounding the clumps. By examining the stability of the clumps, we show that they are gravitationally unbound entities, and that the external pressure from the parental molecular cloud is a significant source of confining pressure for them. Then, the structural transition at \hbox{$A_\mathrm{V}^\mathrm{tail}$}AVtail may be linked to a transition between this population and the surrounding medium. The star-formation rates in the clouds correlate strongly with the total mass in the clumps, i.e., with the mass above \hbox{$A_\mathrm{V}^\mathrm{tail}$}AVtail, and drops abruptly below that threshold. These results imply that the formation of pressure-confined clumps introduces a prerequisite for star formation. Furthermore, they give a physically motivated explanation for the recently reported relation between the star-formation rates and the amount of dense material in molecular clouds. Likewise, they give rise to a natural threshold for star formation at \hbox{$A_\mathrm{V}^\mathrm{tail}$}AVtail.
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