Fragmentation of a gas cloud with orthogonal rotation and magnetic axes

The evolution of a rotating, cool, self-gravitating gas cloud, permeated by flux from the local galactic magnetic field, and with the magnetic and rotation axes mutually orthogonal, is studied. If the flux-to-mass ratio is below a critical value the cloud will contract through a series of quasi-equilibrium states as it loses angular momentum to the surroundings via a centrifugally-driven, magnetically controlled wind. If strict flux-freezing were to hold, the contraction would be essentially isotropic and no fragmentation could occur; but if the contraction is accompanied by significant flux leakage, the cloud flattens parallel to the rotation axis and can split into subcondensations, each with a subcritical flux-to-mass ratio. The process can repeat itself until opaque densities are reached, with the mass M(f) of the final fragments fixed roughly by the flux retained at the epoch when the optical depth reaches unity, rather than by the simple Jeans criterion. If flux leakage is just by ambipolar diffusion, the model sometimes predicts embarrassingly large M(f) values; it is suggested tentatively that in fact there may be a more rapid, dynamical dissipative process, driven by spontaneous hydromagnetic instabilities.