Probing the evolution of molecular cloud structure From quiescence to birth

摘要

Aims. We derive the probability density functions (PDFs) of column density for a complete sample of prominent molecular cloudcomplexes closer than d S 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 mappingtechnique 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 < Av ≤ 3-5 mag, or -0.5 < ln A v /Av ≤ 1). But at higher column densities prominent power-law-like wings are com-mon. 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 onlylow excess of higher column densities. This trend is also reflected in the cumulative forms of the PDFs, showing that the fractionof high column density material is significantly larger in star-forming clouds. These observations agree with an evolutionary trendwhere turbulent motions are the main cloud-shaping mechanism for quiescent clouds, but the density enhancements induced by themquickly become dominated by gravity (and other mechanisms), which is in turn strongly reflected by the shape of the column densityPDFs. The dominant role of the turbulence is restricted to the very early stages of molecular cloud evolution, comparable to the onsetof active star formation in the clouds.