Probing the evolution of molecular cloud structure

Abstract

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 < AV ⪅ 3-5 mag, or -0.5