Ambipolar diffusion and core formation in molecular clouds

The author studies two dimensional axisymmetric models of the quasistatic formation of cloud cores inside a molecular cloud, due to ambipolar diffusion of the magnetic field. He then studies the core evolution with varying amounts of turbulent support, using periodic boundary conditions to represent the formation of molecular cloud cores in a much larger cloud, like the dense cores observed in NH3 in the Taurus molecular cloud by Myers and Benson (1983). He also defines a new critical mass Mcr that includes all available mechanisms of support. A cloud will undergo dynamical gravitational collapse only if the mass M of the cloud is larger than this critical mass. Otherwise, it will evolve quasistatically by ambipolar diffusion of the magnetic field. There are, then, three interesting regimes for star formation: (a) If the cloud is supercritical M/Mcr > 1, gravity can overwhelm all available means of support and the cloud will collapse and possibly fragment into many pieces. If the cloud is subcritical M/Mcr < 1, it will evolve quaistatically as the magnetic field is lost by ambipolar diffusion. Then, small cores will condense from the background and (b) if the core is subumbral, a more condense equilibrium state will be reached with straight field lines and star formation cannot occur. But, (c) if the core is superumbral, there is no equilibrium available and a gravitational runaway will eventually occur: these cores will become dense ammonia cores and finally a low mass star will be formed in the center