The role of ambipolar diffusion in the formation of interstellar cloud cores and protostars

Abstract

The formation of stars is a fundamental unsolved problem in astrophysics. Interstellar molecular clouds, observed to be the birth place of stars, have masses much larger than those of typical stars. Observed densities, temperatures, and strengths of well-ordered magnetic fields imply that magnetic forces, transmitted to the neutral fluid through collisions with ions, dominate thermal forces in supporting these objects against self-gravity. The anisotropy of the Lorentz force, which is always perpendicular to the magnetic field, allows a simplified treatment of the dynamics involving motion along the magnetic field. We derive a reduced system of non-linear, non-ideal magnetohydrodynamic equations by assuming a balance of thermal and gravitational forces along the magnetic field. We then study both the stability of dense clouds as well as their non-linear evolution/ contraction owing to ambipolar diffusion. An important result is the selection by ambipolar diffusion of a characteristic mass comparable to a solar mass.