Numerical simulation of protostellar core collapse

U. Ziegler

Die Beobachtung liefert starke Hinweise dafuer, dass Sterne haeufig als Binaer- oder Mehrfachsysteme entstehen. Die Theorie erklaert dies am plausibelsten anhand der Fragmentation eines gravitativ instabilen, kollabierenden Moelkuelwolkenkerns. Die Untersuchung des Fragmentationsprozesses erfordert die numerische Modellierung der zugrundeliegenden dynamischen Gleichungen (Hydrodynamik, Poissongleichung) auf adaptiven Gittern, um den auftretenden enormen Veraenderungen in der Dichte und Veraenderungen in den Laengenskalen Rechnung zu tragen. Nahezu unverstanden ist der moegliche Einfluss interstellarer Magnetfelder waehrend des Wolkenkollaps -- ein numerisches Problem hoechster Komplexitaet. Mit Hilfe des NIRVANA codes (http://nirvana-code.aip.de) wurden erste prototypische Simulationen in dieser Richtung durchgefuehrt.

Using the NIRVANA code -- state-of-the-art Godunov-type central-upwind scheme; constraint-transport for divergence-free magnetohydrodynamics; multigrid-type Poisson solver for self-gravitating flows; adaptive mesh refinement -- the collapse of a bimodal perturbed solar-mass cloud has been investigated numerically under various assumptions like the type of equation of state of the gas, cloud rotation and the presence of a magnetic field. It has been shown that in the absence of ambipolar diffusion which actually may be ignored only under special circumstances but cannot be treated with the present code version, fragmentation is controlled by the strength and orientation of the applied magnetic field. In case of an isothermal equation of state runaway collapse occurs for both with and without magnetic field and thin (singular) filaments exists as might be expected from theoretical considerations. In case of a barotropic equation of state, however, which mimics the transition from a low-density isothermal state to a high-density adiabatic state of the medium in a more realistic way the dynamical collapse is halted and turns into an accretion phase accumulating matter onto the compact object(s) which develops. The presence of a vertical magnetic field with a mass-to-flux ratio of twice the critical value hereby clearly favors binary formation whereas at the same time for a model without magnetic field a single core emerges which is embedded in a bar and which is surrounded by a ring-like structure (see figs.). Future work aims to include the effect of ambipolar diffusion in order to improve further our understanding on cloud core fragmentation.



caption figs: Density structure with overlain block distribution for the barotropic collapse model with and without magnetic field.