Turbulence in interstellar gas clouds reveals multi-fractal structures

The German-French programme GENESIS (Generation of Structures in the Interstellar Medium) is a cooperation between the University of Cologne's Institute for Astrophysics, LAB at the University of Bordeaux and Geostat/INRIA Institute Bordeaux. In a highlight publication of the journal Astronomy & Astrophysics, the research team presents the new mathematical methods to characterize turbulence using the example of the Musca molecular cloud in the constellation of Musca.

Stars form in huge interstellar clouds composed mainly of molecular hydrogen -- the energy reservoir of all stars. This material has a low density, only a few thousand to several tens of thousands of particles per cubic centimetre, but a very complex structure with condensations in the form of 'clumps' and 'filaments', and eventually 'cores' from which stars form by gravitational collapse of the matter.

The spatial structure of the gas in and around clouds is determined by many physical processes, one of the most important of which is interstellar turbulence. This arises when energy is transferred from large scales, such as galactic density waves or supernova explosions, to smaller scales. Turbulence is known from flows in which a liquid or gas is 'stirred', but can also form vortices and exhibit brief periods of chaotic behaviour, called intermittency. However, for a star to form, the gas must come to rest, i.e., the kinetic energy must dissipate. After that, gravity can exert enough force to pull the hydrogen clouds together and form a star. Thus, it is important to understand and mathematically describe the energy cascade and the associated structural change.