AMBIPOLAR DIFFUSION IN STAR-FORMING CLOUDS

The dynamical collapse of a magnetically supported molecular cloud wit h ambipolar diffusion (hereafter AD) is numerically investigated. Each of the five cloud models considered here has a fixed mass of 1M., a p rescribed outer boundary where the normal derivative of the total pres sure vanishes. In these models, there is no dynamical collapse if AD i s excluded. It is shown that owing to the strong non-linearity exhibit ing the effective AD term, AD appears to be operating in different reg ions with completely different time scales. The models with AD undergo dynamical contraction on a dilute free-fall time scale (tau(ff)) rang ing between 1.1 - 3.0 tau(ff), depending on the ionization fraction in the cloud considered. It is shown that a shock is always formed at th e center which thereafter propagates radially outwards into the high l atitudinal regions leaving quasi ma,onetohydrostatic density profiles behind its shock front. If AD is present, these structures do not surv ive and start to collapse to form a central point mass surrounded by a highly condensed disk-like configuration. Thus, AD is actually able t o change the flow configuration on time scales comparable to fractions of tau(ff). Furthermore, AD appears to give rise to an inside-outside collapse via a magnetosonic expansion wave which starts its motion fr om the center.