Cosmic Magnetism and the Basic Physics of the Early Stages of Star Formation

Abstract

Multifluid magnetohydrodynamics (nonideal MHD) is an indispensable tool for studying the physics of star formation. The physical origin of processes crucial to the formation of stars becomes clear, and the essence of their quantitative theoretical description becomes straightforward when a first-principles approach is taken. Progressively simpler forms of the equations are obtained and used to introduce concepts and describe processes such as ambipolar diffusion and the self-initiated formation and gravitational collapse of fragments (or cores); ohmic dissipation of magnetic flux; flux freezing in the electrons, ions, and neutral particles (through collisional coupling with ions and, possibly, charged grains); quasistatic and dynamical contraction of cloud cores; magnetic braking and the resolution of the thorny angular momentum problem; the magnetic Rayleigh-Taylor (or Parker) instability and formation of cloud complexes; equilibria of isothermal, magnetic clouds; the physical origin of the relation B _c ∝ (ρ_c T _c)^1/2 between the magnetic field strength and the gas density and temperature in cores of contracting, not necessarily isothermal, self-gravitating clouds; and critical masses for the collapse of interstellar clouds. Improvements to the Virial Theorem are also presented. Close contact with observations is emphasized and maintained throughout the paper.

New results from analytical and numerical axisymmetric calculations on the formation and collapse of cloud cores and consequent refinements of a modern, basic theory of star formation are presented in the accompanying paper.