Star Formation and the Interstellar Medium

Interstellar Medium and Molecular Clouds

Turbulence in the Galactic disc can be generated by the magneto-rotational instability (complementary to supernovae driven turbulence).

The image shows the (atomic) gas density in the ISM. It is taken from a simulation in which turbulence is driven by the magneto-rotational instability (Piontek & Ostriker, 2005). The colour-table indicates low density in blue and high density in yellow. The size of each side of the box is 200 pc.

Essentially all known Galactic star formation takes place within molecular clouds, but many of the details of the formation and evolution of these clouds remain obscure, including such fundamental issues as how long they take to form and how long they survive for. Numerical simulations of molecular cloud formation can help to answer some of these open questions. Our simulations are the first to properly combine accurate modelling of the cloud chemistry with a three-dimensional, high-resolution model of the hydrodynamics of the gas in the cloud. Our preliminary work, which focused on the formation of molecular hydrogen, demonstrated that supersonic turbulence plays a crucial role in cloud formation: it dramatically accelerates the rate at which H₂ can form, with the result that the molecular cloud forms within a dynamical time (Glover & Mac Low, 2007). Currently, we are in the process of extending these simulations to model the formation and destruction of the CO molecule, with the goal of producing simulated CO clouds that can be directly compared to observations of CO clouds.

The image shows the (molecular) gas column density in the ISM. It is taken from a simulation in which molecular clouds form in the turbulent ISM. The colour-table is logarithmic in units of particles per square cm. The size of the projection shown here is 20 pc.

SPH simulations of supersonic turbulence and cloud fragmentation suggest that the gravo-turbulent star formation paradigm is able to explain the observed mass distribution of clustered protostellar cores (Jappsen et al., 2005). The assumed gas thermodynamic behaviour has proven to be of particular importance for the resulting core mass function.

The image shows the (molecular) gas column density in the ISM. Black points indicate the collapsed protostellar cores. The image is taken from a simulation of supersonic turbulence and cloud fragmentation. The colour-table is logarithmic and indicates low density with orange and high density with yellow. The size of the projection shown here is 0.3 pc.

SPH simulations of star formation triggered by clump-clump collisions have shown that cloud-cloud interactions at small scales can trigger further star formation (Kitsionas & Whitworth, 2007). Such collisions have proven to be sufficient to explain the observed star formation efficiency in molecular clouds. With such simulations we have also been able to follow the evolution of protostellar discs and their dynamical interactions. Using particle splitting (Kitsionas & Whitworth, 2002) we have been able to resolve the system evolution in these simulations at scales down to ~20 AU (see AIP highlight).

The image shows the (molecular) gas column density in the ISM. Black points indicate the collapsed protostars. The image is taken from a simulation of clump-clump collisions. The filament appearing in the diagonal of the image has formed in the centre of the shocked layer appearing at the end of the simulation. A large circumbinary disc has formed almost perpendicular to the filament following the merger along the filament of two protostellar discs. A tight binary evolves at the centre of the circumbinary disc. The colour-table is logarithmic in units of grams per square cm. The size of the image projection shown here is 0.02 pc. The size of the circumbinary disc is ~1000 AU. The size of the simulation projection is 0.92 pc x 0.56 pc.

Comparisons between numerical codes of hydrodynamic turbulence. We coordinate a comparative study of turbulence decay simulations using different grid- and particle-codes. Our results so far include the calculation of mass weighted power spectra. Further analysis of our results is underway.

The image shows (solid lines) the power spectra obtained at different times of the decay simulation using the PROMETHEUS code of the Wuerzburg group (Germany). The corresponding result of the GADGET SPH code (obtained here at the AIP) is also shown (points).