The link between molecular cloud structure and turbulence

Aims. We aim to better understand how the spatial structure of molecular clouds is governed by turbulence. For that, we study the large-scale spatial distribution of low-density molecular gas and search for characteristic length scales. Methods. We employ a 35 square degrees ¹³CO 1 → 0 molecular line survey of Cygnus X and visual extinction (A_V) maps of 17 Galactic clouds to analyze the spatial structure with the Δ-variance method. This sample contains a large variety of different molecular cloud types with different star-forming activity. Results. The Δ-variance spectra obtained from the A_V maps show differences between low-mass star-forming (SF) clouds and massive giant molecular clouds (GMC) in terms of shape of the spectrum and its power-law exponent β. Low-mass SF clouds have a double-peak structure with characteristic size scales around 1 pc (though with a large scatter around this value) and 4 pc. The GMCs show no characteristic scale in the A_V-maps, which can partly be ascribed to a distance effect owing to a larger line-of-sight (LOS) confusion. The Δ-variance for Cygnus, determined from the ¹³CO survey, shows characteristic scales at 4 pc and 40 pc, either reflecting the filament structure and large-scale turbulence forcing or - for the 4 pc scale - the scale below which the ¹³CO 1 → 0 line becomes optically thick. Though there are different processes that can introduce characteristic scales, such as geometry, decaying turbulence, the transition scale from supersonic to subsonic turbulence (the sonic scale), line-of-sight effects and energy injection caused by expanding supernova shells, outflows, HII-regions, and although the relative contribution of these effects strongly varies from cloud to cloud, it is remarkable that the resulting turbulent structure of molecular clouds shows similar characteristics.