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Probing the evolution of molecular cloud structure (cid:2) From quiescence to birth

Context. Probability distribution of densities is a fundamental measure of molecular cloud structure, containing information on how the material arranges itself in molecular clouds. Aims. We derive the probability density functions (PDFs) of column density for a complete sample of prominent molecular cloud complexes closer than d < ∼ 200 pc. For comparison, additional complexes at d ≈ 250 − 700 pc are included in the study. Methods. We derive near-infrared dust extinction maps for 23 molecular cloud complexes, using the nicest colour excess mapping technique and data from the 2MASS archive. The extinction maps are then used to examine the column density PDFs in the clouds. Results. The column density PDFs of most molecular clouds are well-ﬁtted by log-normal functions at low column densities (0 . 5 mag < A V < ∼ 3 − 5 mag, or − 0 . 5 < ln A V / A V < ∼ 1). But at higher column densities prominent power-law-like wings are common. In particular, we identify a trend among the PDFs: active star-forming clouds always have prominent non-log-normal wings. In contrast, clouds without active star formation resemble log-normals over the whole observed column density range or show only low excess of higher column densities. This trend is also reﬂected in the cumulative forms of the PDFs, showing that the fraction of high column density material is signiﬁcantly larger in star-forming clouds. These observations agree with an evolutionary trend where turbulent motions are the main cloud-shaping mechanism for quiescent clouds, but the density enhancements induced by them quickly become dominated by gravity (and other mechanisms), which is in turn strongly reﬂected by the shape of the column density PDFs. The dominant role of the turbulence is restricted to the very early stages of molecular cloud evolution, comparable to the onset of active star formation in the clouds.