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

Wednesday, 4. November 2015, 16:00 - 17:30
Location Library

N. Schneider: I. Physik. Institut, Cologne, Germany
LAB/OASU Bordeaux, France


Hydrogen column density maps of molecular clouds are one of the most
important observables in the context of molecular cloud- and
star-formation studies. They reflect the structure of the ISM and
constitute the gas reservoir out of which stars form.  Classically,
they were obtained using near-IR extinction and molecular
lines. However, these methods cover a limited range of column
densities and suffer from uncertain conversion factors,
respectively. The Herschel photometric maps (70-500 micron) allow now
to determine column density maps over a very large dynamic range.

Our group of observers and theorists specialized in the last years on
the interpretation of probability distribution functions (PDFs) of
column density.  PDFs are used to evaluate the relative importance of
gravity, turbulence, magnetic fields, geometry, and radiative feedback
governing the cloud's density structure and star-formation activity.
In this presentation, I will summarize our results of comparing
observations with numerical simulations (Schneider et al. 2013;
Russeil et al. 2013; Federrath & Klessen 2013; Tremblin et al. 2014;
Girichidis et al. 2014; Schneider et al., 2015a,b,c,d):

1. For a proper interpretation of PDFs, line-of-sight contamination,
completeness limits, and resolution effects need to be considered. Doing so, we showed that the column
density structure of  quiescent, low-mass, and high-mass star-forming clouds is not the same.
2. By studing column density profiles and infall signatures in
molecular lines, we found clear indications that the power-law tail
commonly found in PDFs is caused by self-gravity of filaments and
clumps, and free-fall collapse of cores.
3. Infrared dark clouds reveal a power-law tail and are thus most likely
dominated by gravity, and not turbulence, even at an early evolutionary state. 4.  The most massive giant
molecular clouds show an additional power-law tail with
flatter slope at the highest column densities, indicating a slowed-down
collapse caused by, e.g., internal stellar feedback, magnetic fields, rotation, or change in geometry. 5. The
PDFs of high-density molecular line tracers such as N2H+ and CS
show a power-law tail that corresponds to the one from dust. However, abundance variations and
different regimes of excitation limit the functionality of molecular line PDFs.


: Nicola Schneider