Star Formation in Magnetic Interstellar Clouds: I. Interplay between Theory and Observations

We summarize the recent interplay between theoretical calculations and observations of interstellar magnetic fields, and the current understanding of the role of magnetic fields in (and a scenario for) star formation. This includes the relation between the magnetic field strength and the gas density in self-gravitating clouds; support of molecular clouds against self-gravity; rotation of clouds and fragments and magnetic braking; molecular line widths and hydromagnetic waves versus supersonic turbulence; core-envelope separation in molecular clouds and the inefficiency of star formation; ambipolar diffusion in cloud cores and thermalization of line widths. A scenario for star formation (binary, single, and planetary) is given which accounts properly for the redistribution of angular momentum by magnetic braking and of magnetic flux by ambipolar diffusion in clouds and fragments; it includes both subAlfvénic (the most common case) and Alfvénic or superAlfvénic collapse, with consequent different efficiencies of star formation in each case. Key problems remaining unsolved are emphasized. The rigorous calculations on which these conclusions are based are summarized in the accompanying paper, where a number of exact mechanical analogies are employed to interpret the MHD solutions physically and where further contact with observations is made.