The structure of molecular clouds

The physics and turbulent properties responsible for shaping the structure in molecular clouds are analysed. Data from the Two-Micron All-Sky Survey are used to determine all-sky median near-infrared extinction maps. Two types of map are made: maps with a near-constant noise in each pixel and maps with a constant resolution. The standard map has a resolution as high as l' along the Galactic plane. The resolution and depth of optical extinction are significantly better than those achieved in previous large-scale studies. The maps are used to examine the column density and mass distributions of 30 nearby molecular clouds. The low column density (i.e. turbulence dominated) parts of the clouds could be well fit by a log-normal distribution. A universal extinction threshold of AV = 5.6 ± 1.2 mag is found, above which there is an excess of material when compared to a log-normal distribution. Gravity is implied as the dominant factor in structure shap­ing above this threshold, as the column density and mass distributions are similar for all clouds. Below this threshold, the structure is determined by turbulence and the environ­ment of the cloud. Two groups of clouds with distinct column density/mass distributions are identified, that correspond to ‘star forming’ and ‘non-star forming’ groups. Structure function and Δ-variance analyses are performed on the same nearby molec­ular clouds. Average values are calculated for the structure function parameters A, intermittency, codimension and fractal dimension. The results imply a mixture of solenoidal and compressive forcing in the turbulence dominated regions of the clouds. An average value for the mass index scaling exponent is calculated which is close to the value expected for clouds where turbulence is driven/governed by solenoidal forc­ing. For the ‘star forming’ clouds the average value implies a mixture of solenoidal and compressive forcing, while the value for the ‘non-star forming’ group implies exclusively solenoidal forcing. It is concluded from this that compressive forcing is associated with the presence of clustered star formation.