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

摘要

How stars form out of their parent molecular clouds remains an unsolved fundamental problem of theoretical astrophysics. Magnetic fields are the dominant means of support against self-gravity and, therefore, important in regulating the rate at which stars form. We formulate the problem of the self-initiated formation and contraction of cloud cores due to ambipolar diffusion in isothermal, magnetic molecular clouds in the presence of MHD waves. The model clouds are initially in exact equilibrium states, with magnetic, thermal-pressure, and wave-pressure forces balancing self-gravity. An energy equation for MHD waves in a partially ionized medium is derived and solved numerically together with the two-fluid MHD equations appropriate for oblate (disklike) clouds about the mean magnetic field. The evolution of the model clouds is initiated by the onset of ambipolar diffusion (the relative drift between neutral and charged particles), which is an unavoidable process in a self-gravitating, partially ionized, magnetic cloud. Redistribution of mass in the central flux tubes of a cloud leads to the relatively slow formation of a magnetically (and thermally) supercritical core, which then begins to contract dynamically while the cloud's envelope remains magnetically supported. We follow the evolution numerically up to central densities of about 3 × 109 cm−3. The MHD waves do not affect the evolution in a significant way, but they are themselves affected by the evolution. We find that the physical processes that affect MHD waves in model clouds are damping by ambipolar diffusion, advection, escape through the cloud surface, energy input from the external medium, and compressive work done by the cloud's contraction. (Shocks are not important for the MHD waves accounted for in this investigation.) One or more of these processes become important at different stages of the evolution. The effect of the wave spectrum on the evolution and vice versa are investigated, as are other free parameters that enter the problem because of the presence of the MHD waves. (The dependence of the solution on the free parameters that appear in the two-fluid, four-fluid, and five-fluid MHD equations in the absence of waves has been previously studied by Mouschovias and coworkers.)