Anchoring magnetic field in turbulent molecular clouds

One of the key problems in star formation research is to determine the role of magnetic fields. Starting from the atomic intercloud medium which has density n _H ∼ 1 cm^-3, gas must accumulate from a volume several hundred pc across in order to form a typical molecular cloud. Star formation usually occurs in cloud cores, which have linear sizes below 1 pc and densities n _H2 > 10⁵ cm^-3. With current technologies, it is hard to probe magnetic fields at scales lying between the accumulation length and the size of cloud cores, a range corresponds to many levels of turbulent eddy cascade, and many orders of magnitude of density amplification. For field directions detected from the two extremes, however, we show here that a significant correlation is found. Comparing this result with molecular cloud simulations, only the sub-Alfvénic cases result in field orientations consistent with our observations.