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 < 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.

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

“…We do not have direct access to the 3D (volume) density from observations-only to the 2D (column) density distribution (Berkhuijsen & Fletcher 2008;Kainulainen et al 2009;Lombardi et al 2011;Schneider et al 2012Schneider et al , 2013Hughes et al 2013;Kainulainen & Tan 2013;Berkhuijsen & Fletcher 2015;Schneider et al 2015). However, one can estimate the 3D density dispersion and the 3D density PDF by extrapolating the 2D density information given in the plane of the sky to the third dimension (along the line of sight), assuming isotropy of the clouds (Brunt et al 2010a(Brunt et al , 2010bKainulainen et al 2014).…”

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

“…PDFs of mass surface density (or N H or A V ) have been used as tools to study the physical characteristics of observed molecular clouds and IRDCs (e.g., Kainulainen et al 2009;Kainulainen & Tan 2013;Butler et al 2014). Mechanisms such as turbulence, self-gravity, shocks, and magnetic fields all contribute to the resulting distribution of Σ.…”

A census of dense cores in the Taurus L1495 cloud from theHerschelGould Belt Survey

“…Those PDFs have been characterised by a lognormal at low column densities which becomes dominated by a power law tail at the high end (Kainulainen et al 2009;Schneider et al 2013Schneider et al , 2015Benedettini et al 2015). The presence of the lognormal component has, however, been questioned by Lombardi et al (2015) who argue that column density PDFs of molecular clouds can be accounted for entirely by truncated power laws.…”