Detection of two power-law tails in the probability distribution functions of massive GMCs

TL;DR

This study detects two distinct power-law tails in the probability distribution functions of high-mass star-forming regions, revealing complex physical processes influencing cloud collapse and star formation.

Contribution

It presents the first detection of a second, flatter power-law tail in the PDFs of massive GMCs, linked to high-mass star formation and cloud core structures.

Findings

First power-law tail consistent with gravitational collapse.

Second flatter tail associated with dense cloud cores.

Excess tail may indicate processes slowing collapse, like rotation or magnetic fields.

Abstract

We report the novel detection of complex high-column density tails in the probability distribution functions (PDFs) for three high-mass star-forming regions (CepOB3, MonR2, NGC6334), obtained from dust emission observed with Herschel. The low column density range can be fit with a lognormal distribution. A first power-law tail starts above an extinction (Av) of ~6-14. It has a slope of alpha=1.3-2 for the rho~r^-alpha profile for an equivalent density distribution (spherical or cylindrical geometry), and is thus consistent with free-fall gravitational collapse. Above Av~40, 60, and 140, we detect an excess that can be fitted by a flatter power law tail with alpha>2. It correlates with the central regions of the cloud (ridges/hubs) of size ~1 pc and densities above 10^4 cm^-3. This excess may be caused by physical processes that slow down collapse and reduce the flow of mass towards higher densities. Possible are: 1. rotation, which introduces an angular momentum barrier, 2. increasing optical depth and weaker cooling, 3. magnetic fields, 4. geometrical effects, and 5. protostellar feedback. The excess/second power-law tail is closely linked to high-mass star-formation though it does not imply a universal column density threshold for the formation of (high-mass) stars.