EPoS Contribution
EPoS Contribution
What probability distribution functions tell us about the processes of star formation

Nicola Schneider
KOSMA, Cologne, DE
Probability distribution functions of hydrogen (column) density (N-PDFs) are known since long as an indispensible tool in theoretical models of star formation. Only recently, they are also employed in observational studies as a convenient and simple to perform analysis method. However, their interpretation and their link to theory is less straightforward. There is a vivid discussion which information on the physical processes governing the density structure and star-formation activity of a molecular cloud are actutally contained in N-PDFs.
I will show that N-PDFs indeed allow to evaluate the relative importance of gravity, turbulence, magnetic fields, geometry, and radiative feedback on the cloud structure. However, for a proper interpretation of N-PDFs, line-of-sight contamination, completeness limits and resolution effects need to be considered and I will carefully outline how these issues can be addressed. Furthermore, I will summarize N-PDF studies using Herschel and molecular line data. The principal outcome is that the observed power-law tail in the high density end of the PDF is caused by gravitational collapse of filaments and cores that are not in hydrostatic equilibrium. The low column density range can be traced down to very small values (visual extinction <<1), and the associated N-PDF is best described as lognormal and thus consistent with a turbulent gas distribution. I will clarify that Infrared dark clouds are gravity - and not turbulence - dominated, and that massive clouds with internal stellar feedback (e.g. ucHII regions, outflows) reveal an additional power-law tail with flatter slope at the highest column densities, indicating a slowed-down collapse. In contrast, large-scale feedback such as an expanding HII region has only a minor influence on the overall column density structure of a cloud. Finally, I will show that N-PDFs obtained from molecular lines are of limited use due to abundance variations and different regimes of excitation.
V. Ossenkopf, U Cologne, DE
R.S. Klessen, ITA/ZAH, DE
T. Csengeri, MPIfR, DE
C. Federrath, Res. School A&A, AU
P. Girichidis, MPA, DE
P. Tremblin, CEA, FR
S. Bontemps, LAB, FR
Suggested Session: Turbulence