Molecular Cloud Formation
Molecular clouds form by the condensation of cold and dense gas out of the warm, diffuse, and magnetised interstellar medium (ISM). This condensation is the result of thermal and dynamical instabilities, which arise due to processes on larger scales, such as gravitational instabilities or collisions of supernova remnant shells. On smaller scales, also turbulence is major ingredient in controlling the formation of dense structures.
In this research project we analyse the formation of molecular clouds by turbulent and magnetised colliding flows and the subsequent star formation process. On the right hand side one can see the formation of a turbulent molecular cloud in the edge-on plane (top) and the face-on plane (bottom). The internal structure of the cloud is filamentary and the filaments and clumps are embedded in a diffuse, warm medium. Since the ISM is subject to heating and cooling processes, these substructures are in pressure equilibrium with the diffuse phase and are thus longlived entities, which eventually collapse to form a star (denoted by the black dots).
Colliding Flows: Face On

coldensflines429B3 Molecular Cloud Formation With Varying Initial Conditions
The above mentioned study assumes a perfect alignment of the colliding flows with the background magnetic field. But this scenario is an idealised one, since turbulence, galactic accretion and stellar feedback drive turbulence on a wide range of scales. Due to the interaction of the turbulent velocity field with the magnetic field, the latter will have a strong non-uniform component. Thus, large scale motions are able to move at an certain angle with respect to the local background magnetic field. If this is the case, the question arises whether molecular clouds are still able to build up and whether star formation is still the outcome.
Our research group aims at understanding the outcome of such non-idealised initial conditions by studying this process with varying inclinations of the flow with respect to the background magnetic field and varying magnetic field strengths.
The picture on the left shows the column density as function of spatial coordinates (in pc) for an initial inclination of one flow of 60 degrees. The blue lines denote magnetic field lines and the black dot represents a sink particle. Click on the image to see a movie of this simulation.

Feedback in Molecular Clouds
Molecular clouds form stars with a broad mass spectrum, which tends to be bottom heavy (i.e. the distribution peaks at low masses) with a power-law tail at high masses. This distribution indicates that massive stars are rare. But despite their low occurrence, these massive stars are the most efficient agents in terms of stellar feedback. Since they are massive enough, they impact onto their parental cloud by means of ionising radiation, fast stellar winds and, finally, by supernovae feedback. During a supernovae, a huge amount of energy is released and injected into the ambient medium in a short amount of time, which is comparable to the overall energy input by the stellar wind. Due to this fact, supernova explosions are thought to disintegrate entire molecular clouds and thus stop the collapse of the cloud and its subsequent star formation process. On the other hand the feedback by massive stars can drive turbulence on molecular cloud scales.
In this project we determine the influence of supernova feedback by massive stars on their parental molecular cloud. In detail we analyse the resulting cloud morphology and dynamics and study the impact on the formation of stars within the densest regions.

Star cluster formation
Here, we study the formation of stars out of a supersonic, turbulent gas core. Seeds of high densities are produced by shocks. These high density regions undergo gravitational collapse and form stars if enough gas is assembled in the shocked regions. Typically, the core fragments into many overdense regions out of which star clusters form.
The animation shows the colum density and the projected star forming regions (black dots).


Jet-driven Turbulence?

Jets and outflows from young stellar objects are proposed candidates to drive supersonic turbulence in molecular clouds. Here, we present the results from jet simulations where we investigate in detail the energy and momentum deposition from jets into their surrounding environment. Our study include jet-clump interaction, transient jets, and magneto jets. We find that collimated supersonic jets do not excite supersonic motions far from the vicinity of the jet. Supersonic fluctuations are damped quickly and do not spread into the parent cloud. Instead subsonic, non-compressional modes occupy most of the excited volume. This is a generic feature which can not be fully circumvented by overdense jets or magnetic fields. Nevertheless, jets are able to leave strong imprints in their cloud structure and can disrupt dense clumps. Our results question the ability of jets to sustain supersonic turbulence in molecular clouds.

Massive star formation via high accretion rates and early disk-driven outflows

We present an investigation of massive star formation that results from the gravitational collapse of massive, magnetized molecular cloud cores. We investigate this by means of highly resolved, numerical simulations of initial magnetized Bonnor-Ebert-Spheres that undergo collapse and cooling. By comparing three different cases - an isothermal collapse, a collapse with radiative cooling, and a magnetized collapse - we show that massive stars assemble quickly with mass accretion rates exceeding 10^(-3) M_sun/year. We confirm that the mass accretion during the collapsing phase is much more efficient than predicted by selfsimilar collapse solutions, i.e. dM/dt ~ c^3/G. We find that during protostellar assembly the mass accretion reaches 20 - 100 c^3/G. Furthermore, we determined the self-consistent structure of bipolar outflows that are produced in our three dimensional magnetized collapse simulations. These outflows produce cavities out of which radiation pressure can be released, thereby reducing the limitations on the final mass of massive stars formed by gravitational collapse. Moreover, we argue that the extraction of angular momentum by disk-threaded magnetic fields and/or by the appearance of bars with spiral arms significantly enhance the mass accretion rate, thereby helping the massive protostar to assemble more quickly.

Supersonic turbulence, filamentary accretion, and the rapid assembly of massive stars and disks

We present a detailed computational study of the assembly of protostellar disks and massive stars in molecular clouds with supersonic turbulence. We follow the evolution of large scale filamentary structures in a cluster-forming clump down to protostellar length scales by means of very highly resolved, 3D adaptive mesh refined (AMR) simulations, and show how accretion disks and massive stars form in such environments. We find that an initially elongated cloud core which has a slight spin from oblique shocks collapses first to a filament and later develops a turbulent disk close to the center of the filament. The continued large scale flow that shocks with the filament maintains the high density and pressure within it. Material within the cooling filament undergoes gravitational collapse and an outside-in assembly of a massive protostar. Our simulations show that very high mass accretion rates of up to 10^(-2) M_sun/year and high, supersonic, infall velocities result from such filamentary accretion. Accretion at these rates is higher by an order of magnitude than those found in semi-analytic studies, and can quench the radiation field of a growing massive young star. Our simulations include a comprehensive set of the important chemical and radiative processes such as cooling by molecular line emission, gas-dust interaction, and radiative diffusion in the optical thick regime, as well as $\Htwo$ formation and dissociation. Therefore, we are able to probe, for the first time, the relevant physical phenomena on all scales from those characterizing the clump down to protostellar core.

Robi Banerjee
Bastian Körtgen
Daniel Seifried (now at University of Cologne)
Marcel Völschow