Detection of two power-law tails in the probability distribution functions of massive GMCs
Résumé
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 (A$_v$) of ~6-14. It has a slope of $\alpha$ = 1.3$-$2 for the $\rho$ $\alpha$ $r^{-\alpha}$ profile for an equivalent density distribution (spherical or cylindrical geometry), and is thus consistent with free-fall gravitational collapse. Above A$_v$~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.
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