Characterizing interstellar cloud turbulence

Turbulence in the interstellar medium produces a cascade ofcomplex, filamentary cloud structures. To deduce the underlyingphysics we need an iterative process of constructing cloud models, comparing their appearance in observable tracers with actual measurements, and improving themodels to resemble the observational results.Each of the steps in the iteration is far from being trivial.Turbulence simulations have to include effects like self-gravity, magnetic fields, local heating and cooling,and various dissipation processes over a range of scales from the sizeof galaxies down to the size of protostellar cores.Promising first steps have been done into this direction.Because the models do not generate an exact representationof the observed interstellar clouds the comparison of the structure between model results and  observationshas to be based on statistical measures. Many tools can becombined. The most simple statistics is given by counting the observedintensity or column density values. The resulting probabilitydistribution functions (PDFs) have a characteristic shape determined by the physical processes driving structure formation in theclouds.  The isotropic scaling behaviour is bestmeasured by means of an optimised Delta-variance analysis. The filamentary properties can be measured by anisotropic waveletsand the velocity structure can be quantifiedby distributions of velocity increments, a velocity drift indexand the smoothness of line profiles. For a complete picture,however, we have to combine the information from multipletracers and from the density and velocity structure. Tools like clump association analysis or scale-depenent cross-correlationfunctions need to be further developped and applied toprovide a better measure for the physical processesdriving and modifying interstellar turbulence. With the current tools it is already possible to constrainsome essential parameters governing the structure of molecular clouds. Continuing this approach will help to recover the nature of interstellarturbulence finally providing a self-consistent picture forthe evolution of interstellar clouds including the complexof star-formation.

Volker Ossenkopf-Okada
KIAA-PKU Auditorium
Yuefang Wu
Thursday, March 9, 2017 - 4:00pm to 5:00pm