No abstract

We present simulations of collapsing 100 M⊙ mass cores in the context of massive star formation. The effect of variable initial rotational and magnetic energies on the formation of massive stars is studied in detail. We focus on accretion rates and on the question under which conditions massive Keplerian discs can form in the very early evolutionary stage of massive protostars. For this purpose, we perform 12 simulations with different initial conditions extending over a wide range in parameter space. The equations of magnetohydrodynamics (MHD) are solved under the assumption of ideal MHD. We find that the formation of Keplerian discs in the very early stages is suppressed for a mass‐to‐flux ratio normalized to the critical value μ below 10, in agreement with a series of low‐mass star formation simulations. This is caused by very efficient magnetic braking resulting in a nearly instantaneous removal of angular momentum from the disc. For weak magnetic fields, corresponding to μ≳ 10, large‐scale, centrifugally supported discs build up with radii exceeding 100 au. A stability analysis reveals that the discs are supported against gravitationally induced perturbations by the magnetic field and tend to form single stars rather than multiple objects. We find protostellar accretion rates of the order of a few 10−4 M⊙ yr−1 which, considering the large range covered by the initial conditions, vary only by a factor of ∼ 3 between the different simulations. We attribute this fact to two competing effects of magnetic fields. On the one hand, magnetic braking enhances accretion by removing angular momentum from the disc thus lowering the centrifugal support against gravity. On the other hand, the combined effect of magnetic pressure and magnetic tension counteracts gravity by exerting an outward directed force on the gas in the disc thus reducing the accretion on to the protostars.

We present 3D "zoom-in" simulations of the formation of two molecular clouds out of the galactic interstellar medium. We model the clouds -identified from the SILCC simulations -with a resolution of up to 0.06 pc using adaptive mesh refinement in combination with a chemical network to follow heating, cooling, and the formation of H 2 and CO including (self-) shielding. The two clouds are assembled within a few million years with mass growth rates of up to ∼ 10 −2 M yr −1 and final masses of ∼ 50 000 M . A spatial resolution of 0.1 pc is required for convergence with respect to the mass, velocity dispersion, and chemical abundances of the clouds, although these properties also depend on the cloud definition such as based on density thresholds, H 2 or CO mass fraction. To avoid grid artefacts, the progressive increase of resolution has to occur within the free-fall time of the densest structures (1 -1.5 Myr) and 200 time steps should be spent on each refinement level before the resolution is progressively increased further. This avoids the formation of spurious, large-scale, rotating clumps from unresolved turbulent flows. While CO is a good tracer for the evolution of dense gas with number densities n 300 cm −3 , H 2 is also found for n 30 cm −3 due to turbulent mixing and becomes dominant at column densities around 30 -50 M pc −2 . The CO-to-H 2 ratio steadily increases within the first 2 Myr whereas X CO 1 -4 × 10 20 cm −2 (K km s −1 ) −1 is approximately constant since the CO(1-0) line quickly becomes optically thick.

We present collapse simulations of strongly magnetised, turbulent molecular cloud cores with masses ranging from 2.6 to 1000 M in order to study the influence of the initial conditions on the turbulence-induced disc formation mechanism proposed recently by Seifried et al. (2012). We find that Keplerian discs are formed in all cases independently of the core mass, the strength of turbulence, or the presence of global rotation. The discs appear within a few kyr after the formation of the protostar, are 50 -150 AU in size, and have masses between 0.05 and a few 0.1 M . During the formation of the discs the mass-to-flux ratio stays well below the critical value of 10 for Keplerian disc formation. Hence, flux-loss alone cannot explain the formation of Keplerian discs. The formation of rotationally supported discs at such early phases is rather due to the disordered magnetic field structure and due to turbulent motions in the surroundings of the discs, two effects lowering the classical magnetic braking efficiency. Binary systems occurring in the discs are mainly formed via the disc capturing mechanism rather than via disc fragmentation, which is largely suppressed by the presence of magnetic fields.