A super-Alfvenic model of dark clouds

Supersonic random motions are observed in dark clouds and are traditionally interpreted as Alfven waves, but the possibility that these motions are su per-Alfvenic has not been ruled out. In this work we report the results of numerical experiments in two opposite regimes: M-A similar to 1 and M-A muc h greater than 1, where M-A is the initial Alfvenic Mach number - the ratio of the rms velocity to the Alfven speed. Our results show that models with M-A much greater than 1 are consistent with the observed properties of mol ecular clouds that we have tested (statistics of extinction measurements, d istribution of integrated antenna temperature, Zeeman-splitting measurement s of magnetic held strength, line width versus integrated antenna temperatu re of molecular emission-line spectra, statistical B-n relation, and scatte r in that relation), while models with M-A - 1 have properties that are in conflict with the observations. We find that both the density and the magne tic field in molecular clouds may be very intermittent. The statistical dis tributions of the magnetic field and gas density are related by a power law , with an index that decreases with time in experiments with decaying turbu lence. After about one dynamical time it stabilizes at B proportional to n( 0.4). Magnetically dominated cores form early in the evolution, while later on the intermittency in the density field wins out, and also cores with a weak held can be generated by mass accretion along magnetic held lines.