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Molecular Line Maps
Modeled PAH Emission
Turbulent Transport

My research interests range around astrophysical gas dynamics, or "IXM-physics", where X stands for the letters (S,G,C). Currently, I am working on the formation of molecular clouds and stars, the role of high velocity clouds for Galactic evolution, how to translate simulation results to the observational plane, and some plasma-physics. This involves numerical simulations on highly parallel machines, such as available e.g. at the NCSA. Astrophysical problems often pose a dire challenge to the numerical scheme. For an appropriate description of simulations, see Cicero.
An artistic view of my work by Jim Cogswell can be found here and here.

Molecular Cloud Formation

Molecular clouds constitute the densest and coldest regime of the interstellar medium. They pervade galactic disks, mostly concentrated in the spiral arms, and they all form stars. Although they only take up a minor fraction of the total galactic mass, they play a crucial role in the galactic evolution. Despite of being regarded as spherical for a long time (partially under the effect of just limited telescope resolution), molecular clouds show an abundance of internal structure, in (column) density, velocity, and as well in magnetic fields (via polarization maps). This wealth of structure points to molecular clouds (and the ISM in general) being highly dynamical, dominated to a large extent by internal turbulent gas motions. This turbulence plays a crucial role in the process of star formation. Where does it come from?

My research helped to identify a simple and intriguing answer: The turbulence arises from the formation process of the cloud. The clouds often are observed to be embedded in converging atomic hydrogen flows. Upon collision, these flows fragment under the effect of strong thermal and dynamical instabilities, converting the highly compressible initial conditions into a close to incompressible turbulent system with high local density perturbations -- ideal initial conditions for star formation (1, 2). The picture above shows a 12CO map of a molecular cloud being assembled by converging flows.

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Star Formation

By now -- especially after the advent of the Spitzer Space Telescope -- it is pretty well established that nearly all molecular clouds form stars. Taken their masses of tens of thousand to a million solar masses and their (average) temperature of approximately 10-20 Kelvin, this is not surprising. On the contrary, one of the oldest problems in star formation theory was to explain why the molecular clouds do not simply collapse under their own weight and convert their mass into stars in one single star burst. Although this type of star formation is seen in extreme environments such as galaxy mergers, local star formation in the Milky Way proceeds much less dramatically: it seems to be highly inefficient. Only a few percent of the cloud mass is converted into stars. The discovery of supersonic turbulent gas motions within molecular clouds promised one way out of the plight: Turbulence was imagined to give rise to a turbulent pressure, much in a similar way as thermal pressure. My research contributed to the efforts to understand in detail the effects of supersonic turbulence within a molecular cloud (3, 4, 5). It turns out that supersonic turbulence has a twofold effect: While it might be able to balance against self-gravity on large scales, and only under favorable circumstances, its local effect is in fact to promote gravitational collapse, due to the compressions caused by the supersonic shocks.

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Observables of the interstellar medium

Especially the more recent large-scale mid-infrared surveys of the Galactic plane (such as GLIMPSE) positively invite to study the turbulent structure and dynamics of the interstellar medium. However, the nature of the mid-infrared emission raises the question how reliable this emission traces (column) density. The main culprit for the observed filigree structure in the mid-infrared seems to be polycyclic aromatic hydrocarbons (PAHs), which are excited by nearby UV-sources and re-emit in the mid-infrared. My research demonstrates how this reprocessing leads to an abundance of structure, in fact, it generates more structure than the underlying gas distribution actually has. Nevertheless, it is possible to extract structural information from the flux density maps -- if one heeds some rules (6)

In the far-infrared, the situation is more beneficial regarding the density information. Here, of special interest is the polarimetry of dense molecular clouds and cores, since it allows to estimate the strength of magnetic fields in such objects (7, 8).

Magnetic polarization vectors over column density map of a self-gravitating rotating core. Rotation is clearly discernible in the field lines. The core is still numerically resolved. Such maps allow a crude estimate of the field strength within the core (7, 8). Column density (top row) of a model molecular cloud, and flux density maps in the mid-infrared. The mid-infrared emission is caused by polycyclic aromatic hydrocarbons, which are excited by the soft UV of the star (visible as intensity maximum in the maps).

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The Fate of High Velocity Clouds

(with M. Putman)
The Galactic halo is teeming with cold clouds of neutral hydrogen, which are known as high velocity clouds (HVCs) because of their inability to fit under any standard model of Galactic rotation. The origin and role of HVCs constitutes one of the major unsolved astrophysical puzzles. HVCs are suspected to provide the gaseous fuel necessary to keep the star formation going in our Galaxy, and they have long been sought after as the probable solution to the G-dwarf problem, or the fact that the metallicity distribution of the long-lived stars in our Galaxy cannot be easily explained without a continuous source of infalling low metallicity fuel.
HVCs are moving through the more diffuse, hot halo medium, as evident from their structure, and from detections of OVI absorption indicating an interaction between HVCs and this medium. The clouds are disrupted by this interaction and form a head-tail structure, i.e. the head of the cloud is compressed around a cold core, and a warmer, diffuse tail extends behind the cloud (see Figure below). It is a matter of debate whether HVCs survive the disruptive instabilities long enough to impinge on the Galactic disk as a ``cold cosmic rain'', implying a very localized fuel source, or whether they are being disrupted before they reach the disk, resulting in a ``warm cosmic rain''. In the latter case, the cloud would be integrated in the warm ionized thick disk of our Galaxy. While also available for feeding star formation, they would be much more easily mixed with higher-metallicity material. Preliminary high-resolution models suggest the second scenario.

Canned High Velocity Cloud

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Turbulent Transport and Reconnection

(with E. Zweibel)
Apart from structuring molecular clouds or possibly triggering star formation, turbulence is believed to have another effect, namely accelerating the diffusion of e.g. chemical species or the dissipation of magnetic field energy over the (otherwise very small) laminar rates. In the context of chemical mixing or heat transport, this problem has been known for a while. However, the effect of turbulence in a weakly ionized medium was less clear. If you would like to learn more about how turbulence affects the magnetic field in a weakly ionized medium, just click on the picture to the right (9, 10).

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(with A. Slyz and J. Devriendt)

Lesser god in Greek mythology, also known as "The Old Man of the Sea". He lives in the sea off the coast of Egypt and can see things in the past, presence and future, but is very unwilling to share his knowledge. In order to evade questions, he has the ability to change his appearance. However, if you manage to catch and hold him, he will assume his true shape and answer your questions.

The above might pertain to any numerical code. Proteus is a 3D gas-kinetic grid code (currently on a fixed cartesian grid), solving the (magneto-)hydrodynamical equations based on the Bhatnagar-Gross-Krook formalism. It is second order in time and space, and allows full control of viscous and resistive dissipation. It is parallelized under MPI. More information can be found here.

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